The War of the Giants was the now almost legendary war homuans fought against a people only remembered as the Giants. The war began before human history, so many of its details, including what started the war, are long forgotten.
What is known is that it last for many years, maybe even more than a century. At the end of it, human civilization as it is understood now came to be in Shijuren. Because of that, the current era is dated metagigantes, or "after the Giants."