Ritter was born in Bad Sooden-Allendorf, the son of a Lutheran clergyman. He was educated at a gymnasium in Gütersloh and at the universities of Munich, Heidelberg, and Leipzig. While studying at Heidelberg, Ritter was a research assistant to the national-liberal historian Hermann Oncken, who a major influence on Ritter. Ritter's first book, 1913's Die preussichen Konservativen und Bismarcks deutsche Politik (The Prussian Conservatives and Bismarck's German Policy) was his PhD dissertation completed in 1911 under the supervision of Oncken. Ritter examined the dispute between Otto von Bismarck and conservative Prussian Junkers who felt that Bismarck's policy was a menace to their traditional privileges in the years 1858-1876. A source of special conflict between Bismarck and the Junkers concerned the latter's opposition to Bismarck's compromises with the southern German states, which were seen as a threat to the traditional powers of Junkerdom. The theme of the extent of one's allegiance to those who hold power would be a recurring subject in Ritter's oeuvre.

From 1912 onwards, Ritter worked as a schoolteacher, and fought as an infantryman in the First World War. Though Ritter was strongly committed to a German victory in the First World War, he was privately bothered by what regarded as the crude chauvinist nationalism of the Pan-German League. Ritter regarded the German defeat of 1918 as a great disaster. This was especially the case because Ritter believed that the monarchy had been the best form of government for Germany, and that the Weimar Republic was a huge mistake because Germany did not have a tradition of republican government. Ritter subscribed to the 19th century view of history as a form of political education for the elite, and contemporary politics were always a pressing concern for him. He married Gertrud Reichardt in 1919, with whom he had three children. Ritter worked as a professor at Heidelberg University, (1918-1923), Hamburg University (1923-1925) and Freiburg University (1925-1956). During his time at Heidelberg, Ritter began an official history of the university from the Middle Ages to the present, of which only one volume was ever published.

In 1925, Ritter published a sympathetic biography of Martin Luther that made his reputation as a historian. In his Luther biography, Ritter treated his subject as an excellent example of the "eternal German". Ritter argued against the view of Luther as an opportunist promoted by Ernst Troeltsch and Max Weber, and instead contended that Luther was a man of faith who possessed the ability to expose what Ritter regarded as grave flaws in the Roman Catholic Church. Ritter argued that Luther inspired his followers to have the self-confidence to improve the world. Ritter's Luther biography was written in large part under the impact of the defeat of 1918, and as such, Ritter went to great lengths to defend what he regarded as the unique German spirit against what Ritter saw as the corrupt materialist spiritual outlook of the West. Throughout his life, Lutheranism was a major influence on Ritter's writings.

In particular, Ritter agreed with Luther's argument that the moral values of Christianity were relevant only to the individual, not the state. Citing Luther, Ritter argued that the state had to hold power, and as part of the messy business of politics, could be guided only by the Christian values of its leaders. Taking up of the ideas of Rudolf Kjelléns and Friedrich Patzel, Ritter argued that the state should be regarded as a living entity, which to live successfully required economic and territorial growth. Using this argument, Ritter contended that Frederick the Great's invasion of Silesia in 1740 was a necessary act to allow the Prussian state to live, regardless of international laws against aggression.

During the last years of the Weimar Republic, Ritter changed his focus from the medieval-early modern periods to the modern period, and from cultural history to biographies of political figures. During this period, Ritter was to write biographies of the Prussian statesmen Karl vom Stein and of King Frederick II of Prussia. Ritter's 1931 two-volume biography of Stein portrayed him as the total opposite of Bismarck. Ritter argued that Bismarck was the ultimate power politician, and Stein was the ultimate anti-power politician. Ritter argued that Stein's success as a politician was limited by his moralism, but contended that despite his lack of political sense was nonetheless successful because of his strong moral character.

On February 11, 1933, in a letter to a friend, Ritter described his intentions as:

"I am planning to write two books. One will be entitled 'What is Liberalism?', and will be the attempt to pave the way for the founding of a large national party of the center, a party which we need today more than ever before. The book will contribute to the drafting of a new liberal national program, which will offer political orientation based on historical reflection...The second book is to...shed light on the great crises in the political and intellectual history of Germany, and will thus explain the present state of mind of the German people. This second book will serve two purposes. It will develop a new concept of the history of our nation...and it will help deepen the notion of the idea of German nationality and national consciousness after a time which this idea has in public use become unbearably trivial. New tasks are crowding in uponous. In our era the historian acquires a distinctive national function, an educational function. Certainly, for the time being no one wants to listen to him, because everyone is still running after noisy political agitators. But I am confident that a time will come when everyone will be thoroughly fed up with the din of national phrase-making and will long for a pure drink instead of the inebriating potion administered by the Nazis. The historian has to prepare positions for the reserves...".

Ritter's 1936 biography of Frederick the Great has been described by the American military historian Peter Paret as one of the finest military biographies ever written. Ritter's emphasis on Frederick's limited war aims, and his willingness to settle for less than he initially sought was seen at the time as a form of oblique criticism of Adolf Hitler. In addition, the emphasis that Ritter placed on the influence of the Enlightenment and "orderly reason" on Fredrick were intended by Ritter to quietly disprove Hitler's claim to be Frederick's successor. The inspiration behind the Fredrick biography was Ritter's personal reaction to the Day of Potsdam of March 22, 1933 where Hitler had laid claim to the Prussian traditions in a way that Ritter felt was not historically accurate.

Ritter was a staunch German nationalist who belonged to a political movement generally known to historians as National conservatism. Ritter identified with the idea of an authoritarian government in Germany that would make his country Europe's foremost power. In an article published in early 1933 entitled "Eternal Right and Interests of the State", Ritter argued that the German people needed most was a government "in which a strong authoritarian leadership will gain voluntary popular allegiance because it is willing to respect eternal justice as well as freedom". Despite severe doubts about the Nazis, initially Ritter reconciled himself to approving of the Nazi regime and its foreign policy, but he broke with the Nazis over the persecution of the churches. In addition, as someone who believed in a Rechtsstaat (Law State), Ritter was opposed to the lawless ways of Nazi Germany. In 1935, Ritter attempted to defend his mentor, Hermann Oncken against attacks by the Nazis who objected to a paper by Oncken which implied the Nazi revolution was not the greatest revolution of all time. In 1938, Ritter delivered a series of lectures in Jena attacking Friedrich Nietzsche, which were intended by Ritter to be a form of indirect protest against the Nazi regime.

Ritter was a devout Lutheran and was a member of the Confessing Church (a group of dissenting Lutherans who resisted the Nazi-imposed "Aryan Christianity") in the 1930s. Ritter belonged to the conservative opposition to the Nazi regime and was imprisoned in 1944-45. In 1938, Ritter became involved in a major debate with Friedrich Meinecke over "Historism". Meinecke argued in favor of the idea of celebrating the "valuable individual quality" of all the phenomenon of history, which was judged not by universal standards, but only in regard to its own values. Ritter attacked this position, arguing that without universal notions of values of good and evil and judging all historical phenomenon by its own standards was to abandon all ideas of morality applicable to all times and places.

In 1938, Ritter was the only faculty member at Freiburg to attend the funeral of Edmund Husserl; his presence at the funeral was widely interpreted at the time and since as an act of quiet political protest against the Nazi regime. After the Kristallnacht pogrom, Ritter wrote in a letter to his mother: "What we have experienced over the last two weeks all over the country is the most shameful and most dreadful thing that has happened for a long time". After the pogrom, Ritter became a founding member of the Freiburger Kreis (Freiburg Circle), a discussion group of anti-Nazi professors which included Adolf Lampe, Constantin von Dietze, Franz Böhm and Walter Eucken. As a result of his underground work, Ritter came to know a number of Catholic and Calvinist members of the German opposition, which caused Ritter to abandon his former prejudices against Calvinists and Catholics. Ritter came to the conclusion that whatever differences divided Lutherans, Catholics and Calvinists, member of three churches had more in common than they did with the Nazis.

In 1940, Ritter published Machtstaat und Utopie (National Power and Utopia). In this book, Ritter argued that democracy was a luxury that only militarily secure states could afford. Ritter argued that because Great Britain was an island, this provided a degree of security that allow democracy. By contrast, Ritter argued that Germany with its location in Central Europe needed an authoritarian government as the only way of maintaining security. In this book, Ritter drew unfavorable comparisons of the utopianism of Sir Thomas More vs. the realism of Niccolò Machiavelli. Ritter argued that for geographic reasons, Germany had to follow the realism of Machiavelli. Ritter argued that were two sorts of values, one Anglo-Saxon and the other continental as personified by More and Machiavelli. Ritter praised Machiavelli as the ideal "continental" thinker who understood the "paradox of power"; namely that power to be effective always involved the use of or threat of violence, but no society could function without power to hold it together. Ritter condemned More for refusing to acknowledge the paradox of power, and instead pretending that morality could function in politics without the threat of and/or use of violence. Ritter went to argue that "continental" thinking about power based upon the understanding of the necessity of violence was much superior to "Anglo-Saxon" thinking which was based on an ineffective legalism. The historian Gregory Weeks commented that it is hard to tell who much of Machstaat und Utopie was material inserted to allow the book to be passed by the censors, and how much was the expression of Ritter's own beliefs, but he has argued that if Ritter was no Nazi, he was certainly a German nationalist who wished to see Germany as the world's greatest power. In a lengthy foot-note to the third edition of Machstaat und Utopie published in 1943, Ritter appeared to disavow his original work of 1940, when Ritter praised More for his understanding of "the demoniacal forces of power" by appealing to Christian morality, and not reducing all politics to "friend-foe" mentality. The historian Klaus Schwable argues that the Ritter's use of the term "friend-foe" was a veiled criticism of Carl Schmitt, who had popularized the term, and was hence an indirect form of political protest.

The Freiburg Circle during World War II wrote a "Great Memorandum" for the proposed post-Nazi German government, which also included "Proposals for a Solution of the Jewish Question in Germany". The "Proposals" rejected Nazi racial theoreis, but stated that the overthrow of the Nazis, German Jews would not have their German citizenship restored, would be restricted to living in ghettos and allowed only minimal contact with German Christians, and called for continuing the Nazi ban on marriage and sex between Jews and German Christians. During this period, Ritter worked as an advisor to Goeredeler on a future constitution after the overthrow of the Nazis. In a Denkscrhift submitted to Goerdeler in January 1943, Ritter wrote that "Hundreds of thousands of human beings have been systematically murdered solely because of their Jewish ancestry." Ritter went on in the same memo to urge that in a future post-Nazi government, that though the Holocaust should be ended, Jews should have no civil rights at all. Ritter was one of the few involved in the July 20 Plot of 1944 who was not liquidated by the Nazis. During World War II, Ritter became involved in work on a study of civilian-military relations in Germany from the 18th century to the 20th century. The original intent behind this work was to offer a critique of the "total war" philosophy of General Erich Ludendorff as a form of indirect protest against the Third Reich. Censorship prevented the book from being published during the war, and after 1945, Ritter revised his work to publish it as a four volume study of German militarism.

Two major themes of Ritter's writings after 1945 were attempts to prove that the Bismarckian tradition in German life had nothing to do with National Socialism and it was democracy of the masses rather aristocratic conservatism which caused the Nazi movement. After World War II, Ritter wrote the book Europa und die deutsche Frage (Europe and the German Question), which denied that the Third Reich was the inevitable product of German history, but was rather in Ritter's view part of a general Europe-wide drift towards totalitarianism that had been going on since the French Revolution, and as such, Germans should not be singled out for criticism. In Ritter's opinion, the origins of National Socialism went back to Jean-Jacques Rousseau's concept of the volonté générale (general will) and the Jacobins. Ritter argued that "National Socialism is not an originally German growth, but the German form of a European phenomenon: the one-party or Führer state", which was the result of "modern industrial society with its uniform mass humanity". Along the same lines, Ritter wrote "not any event in German history, but the great French Revolution undermined the firm foundation of Europe's political traditions. It also coined the new concepts and slogans with whose help the modern state of the Volk and the Führer justifies its existence". Ritter argued that throughout the 19th century, there been worrisome signs in Germany and the rest of Europe caused by the entry of masses into politics, but that it was the World War I that marked the decisive turning point. In Ritter's opinion, World War I had caused a general collapse in moral values throughout the West, and it was this moral degeneration that led to the decline of Christianity, the rise of materialism, political corruption, the eclipse of civilization by barbarism, and demagogic politics that in turn led to National Socialism. In Ritter's view, the problem with the Weimar Republic was not that it lacked democracy, but rather had too much democracy. Ritter argued that the democratic republic left the German state open to being hijacked by the appeals of rabble-rousing extremists. In Ritter's view, had his much beloved German Empire continued after 1918, there would have been no Nazi Germany. Ritter argued democracy was the essential precondition of totalitarianism because it created the window of opportunity for a strongman to make himself the personification of the "popular will", leading Ritter to conclude that "the system of 'totalitarian' dictatorship as such is not a specifically German phenomenon" but rather was the natural result of when "the direct rule of the people derived from the 'revolt of the masses' is introduced". Ritter argued that the precursors of Hitler were "neither Frederick the Great, Bismarck nor Wilhelm II, but the demagogues and Caesars of modern history from Danton to Lenin to Mussolini".

Ritter saw his main task after 1945 of seeking to restore German nationalism against what he regarded as unjust slurs. Ritter argued that Germans needed a positive view of their past, but warned against the appeal of "false concepts of honor and national power". In his treatment of the German Resistance, Ritter drew a sharp line between those who worked with foreign powers to defeat Hitler, and those like Goerdeler which sought to overthrow the Nazis while working for Germany. For Ritter, Goerdeler was a patriot while the men and women of the Rote Kapelle spy network were traitors. Ritter wrote that those involved in the Rote Kapelle were not part of the "German Resistance, but stood in the service of the enemy abroad", and fully deserved to be executed. Besides for defending German nationalism, Ritter became active in the ecumenical movement after 1945, and urged conservative Catholics and Protestants to come together in the Christian Democratic Union, arguing that based on his experience in the Third Reich, Christians regardless of their church needed to work together against totalitarianism. In 1954, Ritter published an acclaimed biography of Carl Goerdeler, a close friend who was executed by the Nazis in 1945. Ritter specialized in German political, military, and cultural history. Ritter always drew a sharp distinction between what he regarded as the Machtpolitik (power politics) of Bismarck where military policy was subjected to carefully limited political goals and the endless expansionism motivated by militarism and bizarre racial theories of the Nazis.

Ritter was well known for his assertions denying that there was a uniquely aggressive German version of militarism. For Ritter, militarism was the "one-sided determination of political decisions on the basis of technical military considerations", and foreign expansionism, and had nothing to do with values of a society. In a paper presented to the German Historical Convention in 1953 entitled "The Problem of Milarism in Germany", Ritter argued traditional Prussian leaders such as Frederick the Great were a Machtpolitiker (power politician), not a militarist since in Ritter's view, Frederick was opposed to "...the ruthless sacrifice of all life to the purposes of war" and instead was interested in creating "...a lasting order of laws and peace, to further general welfare, and to moderate the conflict of interests". Ritter maintained that militarism first appeared during the French Revolution, when the revolutionary French state, followed by Napoleon began the total moblization of society to seek "the total destruction of the enemy". Likewise, Ritter contended that Otto von Bismarck was a Kabinettspolitker (Cabinet politician), not a militarist who ensured that political considerations were always placed ahead of military considerations. Ritter was to expand on these views in a four volume study Staatskunst und Kriegshandwerk (translated into English as The Sword and the Scepter) published between 1954—1968, in which Ritter examined the development of militarism in Germany between 1890—1918. In Ritter's view, in the years following Bismarck's sacking in 1890, that saw the first appearance of militarism in Germany. In Volume 2 of Staatskunst und Kriegshandwerk, Ritter commented that reviewing the first years of the 20th century was "not without a sense of psychological shock". Ritter wrote that "the prewar Germany of my own youth, which has for an entire lifetime been illuminated in my memory by the radiant splendor of a sun that seemed to grow dark only after the outbreak of the war of 1914" was "in the evening of my life" darkened by "shadows that were much deeper than my generation-and certainly the generation of my academic teachers-was able to perceive at the time".

For Ritter, it was the radicalizing experience of the First World War that had finally led to the triumph of militarism in Germany, especially after 1916 when Erich Ludendorff established his "silent dictatorship", which in Ritter's view was a huge break with Prussian-German traditions. It was the unhappy results of that war which finally led to the "proletarian nationalism" of the Nazis gaining a mass audience, and led to the "...militarism of the National Socialist mass movement" coming to power. Moreover, Ritter placed great emphasis on the "Hitler factor" as an explanation for Nazi Germany. In 1962, Ritter wrote that he found it "almost unbearable" that the "will of a single madman" had unnecessarily caused World War II.

Through many regarded Ritter's work as an apologia for German nationalism and conservatism, Ritter was at times critical of aspects of the German past. Through Ritter commented that many nations had bent their knees in submission to false values, that "the Germans accepted all of that with special ardor when it was now preached to them by National Socialism, and their nationalism had in general displayed from its beginning a particularly intense, combative quality". At the first meeting of German historians in 1949, Ritter delivered a speech that declared:"We constantly run the risk not only of being condemned by the world as nationalists, but actually being misused as expert witnesses by all those circles and tendencies that, in their impatient and blind nationalism, have shut their ears to the teachings of the most recent past. Never was our political responsibility greater, not only to Germany, but also to Europe and the world. And yet never has our path been so dangerously narrow between Scylla and Charybdis as today".

In 1953, Ritter found a copy of the "Great Memorandum" relating to German military planning written by General Alfred Graf von Schlieffen in 1905. The following year, Ritter published the "Great Memorandum" together with his observations about the Schlieffen Plan as Der Schlieffen-Plan: Kritik Eines Mythos (The Schlieffen Plan: Critique of An Myth).

In his last years, Ritter emerged as the leading critic of the left-wing historian Fritz Fischer, who claimed that there were powerful lines of continuity between the Second Reich and the Third Reich and that it was Germany that caused World War I. During the ferocious "Fischer Controversy" that engulfed the West German historical profession in the 1960s, Ritter was the best known of Fischer's critics. Ritter fiercely rejected Fischer's arguments that Germany was primarily responsible for the outbreak of war in 1914. The later volumes of Staatskunst und Kriegshandwerk were taken up with attempts to rebut Fischer's arguments. Ritter claimed that Germany did not start a war of aggression in 1914, but rather the German government had merely carried out a foreign policy that contained a high risk of war. Ritter sought to disprove Fischer's thesis by maintaining that the Chancellor Dr. Theobald von Bethmann-Hollweghad attempted to resist the demands by General Ludendorff for wide-ranging annexations as a war aim.

As part of his critique of Fischer, Ritter contended that Germany's principle goal in 1914 was to maintain Austria-Hungary as a great power, and thus German foreign policy was largely defensive as opposed to Fischer's claim that it was mostly aggressive. Ritter claimed that the significance that Fischer attached to the highly bellicose advice about waging a "preventive war" in the Balkans offered in July 1914 to the Chief of Cabinet of the Austro-Hungarian foreign ministry, Count Alexander Hoyos by the German journalist Viktor Naumann was unwarranted. Ritter charged that Naumann was speaking as a private individual, and not as Fischer claimed on behalf of the German government. Likewise, Ritter felt that Fischer had been dishonest in his portrayal of Austro-German relations in July 1914. Ritter charged that it was not true that Germany had pressured a reluctant Austria-Hungary into attacking Serbia. Ritter argued that the main impetus for war within Austria-Hungary was internally driven, and through there were divisions of opinion about the best course to pursue in Vienna and Budapest, it was not German pressure that led to war being chosen as the best option. In Ritter's opinion, the most Germany can be criticized for in July 1914 was a mistaken evaluation of the state of European power politics. Ritter claimed that the German government had underrated the state of military readiness in Russia and France, falsely assumed that British foreign policy was more pacific than what it really was, overrated the sense of moral outrage caused by the assassination of Archduke Franz Ferdinand on European opinion, and above all, overestimated the military power and political common sense of Austria-Hungary. Ritter felt that in retrospect it was not necessary from the German point of view to maintain Austria-Hungary as a great power, but claimed that at the time, most Germans regarded the Dual Monarchy as a "brother empire", and viewed the prospect of the Balkans being in the Russian sphere of influence as an unacceptable threat. Ritter argued that though the Germans supported the idea of an Austrian-Hungarian invasion of Serbia, this more of an ad hoc response to the crisis gripping Europe as opposed to Fischer's claim that Germany was deliberately setting off a war of aggression. Ritter accused Fischer of manufacturing the quote he attributed to the German Chief of the General Staff Moltke during a meeting with the Austro-Hungarian War Minister, Field Marshal Conrad von Hötzendorf about the necessity of a "speedy attack" on Serbia. Ritter claimed that importance that Fischer attached to the report of the German Army's Quartermaster that the Army was "ready" for war in 1914 was mistaken since the Quartermaster always reported every year that the Army was "ready" for war. Likewise, Ritter claimed that the order by Dr. Bethmann Hollweg to the Count Roedern, the State Secretary for Alsace-Lorraine, to put a stop to Francophobic remarks in the German-language press in Alsace was proof of Germany's desire not to have a wider war in 1914, and claimed that Fischer's contrary interpretation of Dr. Bethmann Hollweg's order not to be supported by the facts. Contrary to Fisher's interpretation, Ritter maintained that Bethmann Hollweg's warnings to Vienna were meant to stop a war, and were not window dressing intended to distract historical attention from German responsibility for the war. Ritter claimed that Fisher's interpretation of Bethmann Hollweg's meeting with the British Ambassador Sir Edward Goschen was mistaken since in Ritter's opinion, if Bethmann Hollweg was serious about securing British neutrality it made no sense for expressing the imperialistic war aims to Goschen that Fischer credited him with. Ritter strongly disagreed with Fischer's interpretation of the meeting of Moltke, Bethmann Hollweg and the Prussian War Minister, General Erich von Falkenhayn on July 30, 1914. Rather than a conscious decision to wage an aggressive war as Fischer argued, Ritter claimed that it was the news of the Russian mobilization that led the generals into persuading a reluctant Bethmann Hollweg to activate the Schlieffen Plan. Ritter was strongly critical of what he regarded as Fischer's "biased" view of Moltke's reaction to the outbreak of the war, and argued that Moltke's opposition to the sudden last minute suggestion of Wilhelm II that the German attack on France be cancelled was due to logistical concerns rather than a desire to provoke a world war. Finally, Ritter complained that Fischer relied too much on the memories of Austro-Hungarian leaders such as the Count István Tisza and Count Ottokar Czernin who sought to shift all of the responsibility for the war on German shoulders.

Furthermore, Ritter argued there were no lines of continuity between the Second and Third Reichs and considered the Sonderweg view of German history a myth. Ritter claimed it was not true as argued by Fischer that both world wars were "wars for hegemony" on Germany's part. In 1964, Ritter successfully lobbied the West German Foreign Ministry to cancel the travel funds that had been allocated for Fischer to visit the United States; in Ritter's opinion, giving Fischer a chance to express his "anti-German" views would be a "national tragedy", and it was the best that Fischer not be allowed have his American trip. Writing in 1962, Ritter stated he felt profound "sadness" over the prospect that the next generation of Germans would not be as nationalistically-minded as previous generations as a result of reading Fischer.

In 1959, Ritter was elected an honorary member of the American Historical Association in recognition of what the A.H.A described as Ritter's struggle with totalitarianism. Ritter was the fifth German historian to be so honored by the A.H.A. He died in Freiburg. Ritter was one of the last of the traditional German Idealist historians who saw history as an art, concerned themselves with imaginative identification with their subjects, focused on the great men of the times under the historian's study, and were primarily concerned with political and military events. Despite his numerous books and leading position after World War II, Ritter is almost forgotten in German historiography today, while Fischer's works inspired a new generation of younger German historians.