The Titanomachy was a ten-year series of battles fought in Thessaly, consisting of most of the Titans (an older generation of gods, based on Mount Othrys) fighting against the Olympians (the younger generations, who would come to reign on Mount Olympus) and their allies. The stage for this important battle was set after the youngest Titan, Cronus, overthrew his own father, Ouranos, with the help of his mother, Gaia.
Ouranos drew the enmity of Gaia when he imprisoned her children the Hecatonchires and Cyclopes in Tartarus. Gaia created a great sickle and gathered together Cronus and his brothers to convince them to castrate Ouranos. Only Cronus was willing to do the deed, so Gaia gave him the sickle and placed him in ambush. When Ouranos met with Gaia, Cronus attacked Ouranos, and, with the sickle, cut off his genitals, casting them into the sea. His father, however, cursed Cronus and told him that he is fated to be overthrown by one of his own children as he did to him.
Cronus took his father's throne after dispatching Ouranos. He then secured his power by re-imprisoning his siblings the Hecatonchires and Cyclopes in Tartarus. Cronus, paranoid and fearing the end of his rule, now turned into the terrible king his father Ouranos had been, swallowing each of his children whole as they were born from his sister-wife Rhea. Rhea, however, managed to hide her youngest child Zeus, by tricking Cronus into swallowing a rock wrapped in a blanket instead. Rhea brought Zeus to a cave in Crete, where he was raised by Amalthea. Upon reaching adulthood, he masqueraded as Cronus' cupbearer. Once Zeus had been established as a servant of Cronus, Metis gave him a mixture of mustard and wine which would cause Cronus to vomit up his swallowed children. After freeing his siblings, Zeus led them in rebellion against the Titans.
Zeus then waged a war against his father with his disgorged brothers and sisters as allies: Hestia, Demeter, Hera, Hades, and Poseidon. Zeus released the Hecatonchires and the Cyclopes from the earth (where they had been imprisoned by Cronus) and they allied with him as well. The Hecatonchires hurled stones, and the Cyclopes forged for Zeus his iconic thunder and lightning. Fighting on the other side allied with Cronus were the other Titans with the important exception of Themis and her son Prometheus who allied with Zeus. Atlas was an important leader on the side of Cronus. The war lasted ten years, but eventually Zeus and the other Olympians won, the Titans were imprisoned in Tartarus, and the Hecatonchires were made their guards. Atlas was given the special punishment of holding up the sky. In some accounts, when Zeus became secure in his power he relented and gave the Titans their freedom.
Following their final victory, the three brothers divided the world amongst themselves: Zeus was given domain over the sky and the air, and was recognized as overlord. Poseidon was given the sea and all the waters, whereas Hades was given the Underworld, the realm of the dead. Each of the other gods was allotted powers according to the nature and proclivities of each. The earth was left common to all to do as they pleased, even to run counter to one another, unless the brothers (Zeus, Poseidon and Hades) were called to intervene.