Sometimes a war is justified, and coming to the aid of an ally getting invaded is a damn good justification.
Especially if that ally is simply asking for more hardware and not asking for feet on the ground.
Edit: Fixed the link, it was broken for some reason though it worked earlier today. IDK this mirror should work though
A war is always a good idea to reclaim lost territory. (Unless you fail)
I'm reasonably confident that every part of the inhabited world has been "lost territory" more than once.