Who are the Titans? Why did they ancient Greeks believe the Titans lost power?

The Titans where the creators and rulers of the world before the Olympic Gods came to power. So basically before the gods the world was ruled by huge giants with powers of their own. For example Chronus or Kronos was the titan of time and the ages. He was father to the gods Zeus, Hades, and Poseidon. Something happened and some prophacy or something thing warned him that he would be replaced as king of the Gods or in his case the titans by one of his children. So he chose to devour them (swallowed them whole). But Rhea, Kronos' wife and mother to the Olympians hid her son (Zeus) and raised him to adulthood in secret and had him train to kill his father and release his brothers; Hades and Poseidon from Kronos' stomach. He did and the three joined together and defeated their father and basically went all out and got rid of all the titans and created Olympus; A nountain in which the gods lived at the top of. They had children and such and grew in number.