The End War was the event that led to the destruction of The Vanguards. The war started when Purists created a portal (now known as the end portal) to travel to a new dimension. Once there, they were subsequently killed by endermen and were never heard from again.
A second party went back to find them and suffered heavy losses but managed to kill the end dragon and return home to Allimore. Unfortunately, they neglected to shut of the portal and endermen came through, back to their world. They killed every vanguard in sight and for the next year, continued to hunt every last one down.
Some Purists were able to fight back, but the endermen had limitless numbers and easily overran the Vanguard race.
The War Ends Edit
The war ended when the final Purist left his community of surviving Vanguards to put a stop to the endermen. It is unknown what he did exactly but it expelled most of the remaining endermen back into the end. Now a few remain, and are passive to anyone who doesn't look them in the eye.
The war brought an End to the Vanguard empire, which is why we call it the End war. We now know why endermen roam the land, they don't quite match with any other creature.