Imagine a world in which the gods directly intervened, in which a prayer to Neptune, Lir, Manannan or Aegir might sink an invasion fleet. Imagine that our world was like that in ancient times but that the gods have since withdrawn. Poul Anderson wrote some works of fiction in which that was literally true and others in which it was believed to be true. I have catalogued his volumes set in the past before and it is an impressive list:
The Last Viking
The Fourteenth Century
The Time Patrol
In haste. The weather has improved and we are at last going out for the afternoon.