I was playing as Britannia in Rome total war and ended up at war with Rome when they attacked a settlement of mine. I got a full stack to back up settlement which also had a full stack base, as soon as the deployment was sorted the Roman army just walked off without a single arrow or javelin being fired. Then I get a message before the end of the battle "Your reinforcements are not joining you" it annoyed me but I didn't need them anyway. It's the 2nd time the AI just walked off before a battle started.