Which makes a lot of sense. If you could wait until you need backup to join, you'd just never join until you need it. No country wants to get sucked into someone else's war.

At least, that's the mentality. The truth is, no war is "someone else's war". We're all in this together globally, and oppression anywhere is a threat to everywhere.

There is no such rule. The rule is everyone else already in has to agree to you joining. Practically most people don't want to go to war and so nobody will agree, but there is no rule stating they can't agree.