The Battle with the Titans
With his rescued siblings, Zeus had the beginnings of an army with which to challenge Cronus. However, Cronus had some difficulty in assembling his own forces. Some of the Titans refused to help him in the struggle. None of the Titanesses participated, and Oceanus, Cronus's brother, also refused to fight. Similarly, Helios, son of Hyperion, refused to take part in the war. Prometheus and Epimetheus, sons of Iapetus, blatantly refused to pledge loyalty to Cronus; rather, they eventually sided with Zeus's army. The remaining Titans chose Atlas, another son of Iapetus, to lead them into battle.
Prometheus possessed the gift of prophecy, which is why he pledged his loyalty to Zeus. Prometheus knew what was coming and used that knowledge to offer advice to the Titans. When the Titans disregarded him, Prometheus joined the Olympians; when the battle was over, he wanted to be on the winning side.
In preparation for war, each side created fortifications. Led by Atlas, the Titans gathered at Mount Othrys; the children of Cronus, under Zeus, gathered at Mount Olympus.
The war was a monumental conflict. The Titans were awesome creatures who possessed considerable strength. The children of Cronus were just as strong and cunning. The two sides met on the battlefield every day for ten long years, each side winning some battles and losing others. After ten years, however, the war was no closer to a decisive victory. So Gaia, who knew a thing or two about overthrowing one's father, interceded and advised Zeus.
Gaia told Zeus that freeing the Cyclopes and Hecatoncheires from Tartarus would gain the Olympians some very powerful allies. Zeus wasted no time. He ventured into the depths of the Underworld and faced Campe, a monster appointed by Cronus to guard the giants. Zeus slew Campe and freed his uncles. As Gaia had predicted, the Hecatoncheires and the Cyclopes were so angry with Cronus for his treatment of them that they joined forces with the Olympians.
With these giants newly recruited to Zeus's army, the tide of the war began to turn. The Cyclopes built impressive weapons, including lightning, thunder, earthquake, a trident, and a helmet of invisibility. The Hecaton-cheires threw great boulders at the Titans' fort, weakening it.
Zeus laid siege to Mount Othrys. But strength alone would not win the war, so he devised a plan to force Cronus's army to surrender. Using the helmet of invisibility, one of the Olympians walked into the camp unnoticed and stole all of Cronus's weapons. Another Olympian distracted Cronus with the trident while Zeus shot lightning bolts. Meanwhile, the Cyclopes and Hecatoncheires rained boulders down upon the Titans. Zeus's strategy succeeded, and the war that had nearly destroyed the universe was finally over.