Gods vs. Titans
Cronos received a prophecy that he would be defeated by his son, so he ate all his children. His wife, Rhea, didn't like that he was eating all their children, so she bore three children in secret. These children (Zeus, Hades, and Poseidon) came back after they grew up, freed their devoured brothers and sisters, and overthrew the Titans.
Lapiths vs. Centaurs
So the king of the Lapiths, Pirithous, was getting married to Hippodamia, and he invited the centaurs because he was related to them. The centaurs soon got drunk and tried to kidnap Hippodamia, so a battle broke out and the Lapiths won.