What's really funny is when an ally has a big city of theirs that is under siege, I have a huge stack right on the border, and they still refuse military access. My troops had an excellent view of them losing their city

Unfortunately I had to divert those troops elsewhere at Senate request, but another side benefit of this experience is that I can now take that same city later w/o my ally getting mad at me. If I'm in a good mood when I do it, I might even give it back to them ... for a price ...