If you are an ally of a defender your part in it only lasts a week and then you need to re-ally. that’s kind of mindblowing, and useful all in itself (that you’re not stuck in if it goes bad)
eve university doesn’t mention this - Corporation diplomacy - EVE University Wiki
ccp obviously didn’t mention it (i guessed) - Wars – EVE Online
the forums didn’t mention it (troll cesspool) - Search results for 'war defender ally' - EVE Online Forums
i asked in help channel nobody knew wtf i was talking about
here’s the picture of what it looked like when i made the video
defender’s ally - https://i.imgur.com/e39VmJQ.png
CAVEAT: video was made when i thought i found a weird bug, and possible exploit for ending a war prematurely. this forum post here in the general discussion is my followup to that.