I was just posting this to kind of open up the topic on when your empire falls. In team games, I get really annoyed when team members fall and dont do anything, so im posting this to let you know what to do.
Should an ally come under attack, send a small force to see if you cant sway the tide of battle, and if that doent work, then dont do anything more. If you send your main army, they will be noticed going, and then the other enemy(presuming a 2v2, or 3v3 game) will just eliminate you. Rather, if you or your ally is being overcome, the one who is being destroyed shouldnt panic and begin building units. it wont happen. instead, pay tribute. have the empire that is dying pay tribute of all of its resources to its ally. then, the remaining ally will have enough resources to successfully protect itself and have the best chance of winning.