The Three Big Gods: A Pact After World War II

Zeus, Poseidon and Hades all made a pact after World War II. Their children were just too powerful to sire anymore heroes. It is one of the only times there's maybe one other beat in history that the book deals with what these gods have been doing since the age of mythology. Almost everything the age of the talk about like a literal centaur. So Percy Jackson's teacher ends up being the centaur Chiron who's also protecting him.