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.
> but probably reasonably frequently the case that war is the only viable choice for one side
I think this is untrue... unless your invaders are the assyrians, surrender and losing the war isn't really that bad. Even the Mongols would just collect some extra taxes.
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.