I wouldn't say there HAS to be a war going on. After all, there was no war in Predators, and to be honest even in the original the war was kinda over once Arnie and his boys had killed absolutely everybody.
In my eyes the conflict thing is one of the more flexible aspects of the formula; the Predators may like the extra challenge that a surrounding conflict brings, and it would certainly help mask their activities, but if the setting was remote enough that they wouldn't be discovered I can see no reason why they wouldn't just stalk a bunch of people that weren't up to their neck in warfare. Still fits the idea to me.