Add Diplomatic option: Ask faction to leave another alliance

I like this. It could also work in the opposite way; having factions ask you to leave your alliance. I know I've been allied in a group, only to have other factions sending me the "we should consider an alliance" message, but never sending the actual prompt because I was already allied with someone else. 

Where this idea could get really fun, is having a bidding war. Have the different factions submit offers, choose whomever is willing to offer the most to have your aid.