The novel “Lest Darkness Fall” by L. Sprague de Camp was published for the first time in 1941.
Martin Padway is an American archeologist visiting Rome. One day in 1938 he’s visiting the Pantheon when a strong storm starts and is almost hit by a lightning strike. Stunned, he slowly becomes aware that the people around him are strangely dressed and speak in an almost incomprehensible manner in a language other than Italian.
Recovered from dizziness, Martin Padway tries to figure out what happened and, asking for the date in the Latin spoken by the people he meets, finds out he’s in Rome in 535 A.D. during the reign of King Thiudahad. He has no way of returning to his time so he has to try to survive by exploiting his modern knowledge with the awareness that Italy is about to be invaded by Emperor Justinian’s army.