After having a non aggression pact broken with an AI I was unable to use 'Under One Banner' on one of their outposts.

The border around the outposts appeared as though we were allied (laurel leaves) despite the crest having thorns around it in diplomacy at top of screen. AI broke pact by attacking my army and diplomacy screen indicates we are now at war. 

When I attempt to use Under One Banner (roman culture) the area around the outpost becomes red and if I click messages comes up telling me that I can't use it on an Ally or someone I have a non aggression pact with. 

Sorry I can't put screenshots up as I am on a different PC. Can add later today if it helps.