Used in online games, or shooting activites such as paintball.
It refers to when two opposing people on different team (or potential team-mates) start a fued caused by an angry experience shared between them and purposefully aim for or anger each other further. The word Interwar derives from the term "Internet War"
E.g. Counter Strike
Terrorist Max: Oi, there's no way you could've head shotted me there! Hacker!
Counter-terrorist Tom: Hey, I don't hack, i'm just good.
Terrorist Max: I'm coming for you!
This is where the Interwar starts. When two opposing forces collide and as previously mentioned, usually in a fight or dangerous experience.