The Only War was a war fought between The Imperium and the High Empire - Multiverse-spanning empires with enough power to cause mutual destruction and probably the destruction of the Multiverse with them. Ameryl Hypericum and the Highemperor agreed to a peace treaty that would make this the only war between their domains and the two entered a cold war state. The two empires since have had both hot and cold relations as they both help and hinder each other as the mood fits.
Under the threat of mutual destruction and the destruction of the Multiverse, the two sides agreed to halt hostilities and enter a cold war state. Since then The Imperium has been known to help the High Empire[Tales 1] and also do minor battle with them[Tales 2].