The First Olympian War is also known as Titanomachy. It was the ten year series of battles fought between the two races of deities towards the begining of mankind's existence: the Titans, fighting from Mount Othrys, and the younger generation of gods, who would come to reign on Mount Olympus.
Uranus drew the enmity of Gaia when he imprisoned her children the Hekatonkheires and Cyclops in Tartarus. Gaia created a great sickle and gathered together Kronos and his brothers to persuade them to castrate Uranus. Only Kronos was willing to do the deed, so Gaia gave him the sickle and placed him in ambush.
When Uranus met with Gaia, Kronos attacked Uranus with the sickle and cut off his genitals, and cast the severed member into the sea. From the blood that spilled out from Uranus and fell upon the earth, the Gigantes, Erinyes, and Meliae were produced, and from the spume from his cut genitalia, Aphrodite rose from the sea. Kronos took his father's throne after dispatching Uranus. He then secured his power by re-imprisoning his siblings the Hekatonkheires and Cyclops, and his (newly-created) siblings the Gigantes, in Tartarus.
Gaia, angry at Kronos for keeping his brothers in the same pit Uranus locked them in, made a prophecy that Kronos' own children would rebel against his rule just as he had done to his own father. For fear of his unborn children rising against him, Kronos now turned into the terrible king his father Uranus had been, swallowing each of his children whole as they were born from his sister-wife Rhea. Rhea, however, managed to hide her 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 to adulthood. Later, Metis gave Zeus a mixture of mustard and wine for Kronos which would cause him to vomit up his swallowed children. Zeus then led his released brothers and sisters in rebellion against the Titans.
Then Zeus released the brothers of Kronos, the Gigantes, the Hekatonkheires, and the Cyclops, from their dungeon in Tartarus, killing their guard Kampê. As a token of their appreciation, the Cyclopes gave him thunder and the thunderbolt, or lightning, which had previously been hidden by Gaia. Together, Zeus and his brothers and sisters, along with the Gigantes, Hekatonkheires and Cyclopes overthrew Kronos and the other Titans, in the combat called the Titanomachy. The defeated Titans were then were put away. Atlas, one of the Titans that fought against Zeus, was punished by having to hold up the sky.
The war between God and Titans lasted for ten years, until the Olympians claimed victory. Zeus fought Kronos in the final battle, slicing the Titan lord to pieces with his own scythe and sending him to the Underworld.
After reaching manhood, Zeus and Metis gave Kronos an emetic to force him to disgorge the babies.
After the battle with the Titans, Zeus shared the world with his elder brothers, Poseidon and Hades, by drawing lots: Zeus got the sky and air, Poseidon the waters, and Hades the world of the dead (the underworld). The ancient Earth, Gaia, could not be claimed; she was left to all three, each according to their capabilities, which explains why Poseidon was the "earth-shaker" (the god of earthquakes) and Hades claimed the humans that died.
Gaia resented the way Zeus had treated the Titans, because they were her children. Soon after taking the throne as king of the gods, Zeus had to fight some of Gaia's other children, the monsters Typhon and Echidna. He vanquished Typhon and trapped him under a mountain, but left Echidna and her children alive, as tests to future heroes
The result of the First Olympian War was a change in command. Zeus elected himself king of the gods, and the Titans that fought against him were put in various prisons.
Humans began to develop under Zeus' rule, and life was prosperous.