For the 2nd game in a row now I have had defence pacts with all the civs except the most aggresive. Doing so to help keep them at bay until I can build up my own forces. Everytime they, the aggressor, attack me noone comes to my aid. But as soon as someone gets attacked I am immediately in a war with the biggest military force on the map. Am I missing something here? Is there some trick to having these pacts? Or are they just not worth the effort for the human player?