Was just playing a game where the Coalition offered me a Defence Pact for 30 Turns. Of course I signed it as they were my biggest neighbour and I was making over 1000 credits in trade with them [Heyoun/Drizzi/Brac ftw] The turn I accept it they declare war on me. So they send out their ships to attack my planets, but I can't attack them as I have a Defence Pact still. Crafty hew-mons.

Thanks for reporting this bug. I fixed it (hopefully)