Lest Darkness Fall Author:L. Sprague De Camp This story, in a shorter version, appeared in the December, 1939 issue of Unknown. This sentence is on the page with the copyright info but no ISBN number is listed. — From Cover: — Moments ago, Martin Padway had been hurtling through Rome's murderous traffic, clinging to his seat in a little fiat and trying to pay attention as Professor Tancredi ... more »expounded upon his theory of time.
The professor's idea was that history was a four-dimensional web with weaknesses at key junctures-places like Istanbul or Rome, where the lines of great events intersect. People who vanished without a trace might actually have slipped backward in time.
It had all seemed innocuous. Then Tancredi dropped Martin off at the Pantheon, a raindrop hit his hand, thunder rumbled overhead,lightning struck...and the pavement dropped away beneath him.
When Martin looked up, blaring car horns had given way to the creaking of oxcarts. Martin was in ancient Rome, living proof of Tancredi's theory
This classic of alternate history, about a modern-day man suddenly transported to late Imperial Rome-and determined to stave off the onset of the Dark Ages-is as fresh, witty and full of surprises as it was 55 years ago.« less
Not at great example of this sub-genre, but it is entertaining and more realistic. I checked on Fantastic Fiction and this novel is one of the first three books he wrote, in 1941.