The titans won.
They won the war because they were too powerful. They all awoke from their sleep on the last day it wasn't just Cronus, it was everyone. When they attacked, all at once, there was no one to stop them. They won the war. They destroyed the world in the process.
Every single continent was made the home of titans with need for destruction. They became completely uninhabitable. But Cronus was the smart one who settled in what was left of Europe, the last place that the titans attacked—the original home of the gods. He settled there and created his new home, a place known as Panem.
To kill the gods would be too easy for Cronus's taste so he bound them to 13 areas, the 13 Districts. Their offspring was put in these districts, mingled with what was left of the normal population. The gods were finally divided and were thus robbed of the power they gained through their unity.
The 13th district though, consisting of all the minor gods of the pantheon, stayed powerful enough to defy Cronus. But without the help of the major gods and their children, they were far from powerful enough to defeat Cronus. The entire District 13 was eliminated, eradicated, destroyed from the face of this Hell of an Earth. The demigods of District 13 were killed mercilessly and their parents were devoured by Cronus.
Cronus then devised a simple plan. In penance for their uprising, each district would offer up a male and female demigod between the ages of 12 and 18 at a public "Reaping". These tributes would be delivered to the custody of The Capitol and then transferred to a public arena where they will Fight to the Death, until a lone victor remains.
Henceforth and forevermore this pageant would be known as The Hunger Games.