I think it's quite rare that war is the only viable choice for both sides, but probably reasonably frequently the case that war is the only viable choice for one side. Sort of like how many crimes require both a victim and a perpetrator, but only one side is making the choice.

It's also the case that in many modern wars both sides will claim to be the side with no choice though. Also that having entered a war by choice doesn't necessarily imply that it's still possible to exit the war by choice.

It's not just the only choice, but a phenomenon that eventually gets a mind of its own. There is a point in almost every war where both sides regret the escalation, but are powerless to stop the war as the time tables have been set, the troops mobilised, the ultimatums issued. And the belligerents, deeply invested in the outcome, cannot stop the war before the cataclysmic defeat of one side.

The inevitability of war beyond a certain point is a terrifying prospect.

It is thick, but wilfully. I think you (or autocorrect) mean vicious.