The Aertian War is the longest and bloodies war in the history of the Imperium. It was a war over Imperium rule, and the the self proclaimed independence by the newly name Western Free Cities. When the Imperium refused to allow the Free Cities to govern themselves, a long and bloody conflict began. The biggest victims in the war were the Orcish tribes. Forced to choose a side, they fought with the West, and made up a large part of their forces.
Orcs in this world, though violent and brutal, and prone to conflict, are not completely uncivilized and proved themselves to be loyal allies, despite their heavy losses.
20 years ago, an armistice was brokered in the old Imperium city of Elders Bay. Elder Baka Roosk met with the appointed leaders of each of the six western free cities, and after almost a month of negotiation, the war was essentially over.
I say essentially, because there was never a formal surrender by either side, just an end to the fighting. Elder Roosk worked hard to keep the peace after that, acting as negotiator and ambassador to the west.
The only group that never saw the armistice as an end to the war, were the Orcs. Now, forced to retreat to the lands now known as the Orc's Arrow, they continue their war against their old enemies in the east, and what they see as their new enemies in the west. They never forgave the Western Free Cities for the armistice, after they gave so much for the war.
Baka Roosk wrote on this, in the formal armistice:
"Of all the evil this war has born, I regret nothing more than the devastation caused to the Orcish people. Our clerics estimate that they will live no more than a few generations, such was the devastation to their number."
To this day, Orcs commit to raiding both Imperium and Western lands when they can, though they are unable to muster more than the smallest fighting force, as most of the tribes are at war with each other these days.