This is one of the oldest wars in the universe.
The war begain around mid-evil times. During this xenomorphs wanted conquest of the universe. But by this time the predators begain hunting the xenos. And soon on predator's hunting world and planets, xenos soon begain to die more in conflects in the's hunts and would soon be treated like animals. And of course it pissed of most of the xenos. The conflects turned it to battles, which turned in to a war. The war ended when the hunters killed of most of the great hive mines which ruled over all xenos.