Like Teutonic said, you can uncheck the box to ask your allies to join you, and then one turn later you can generally get a ceasefire with the enemy's allies pretty easily. That's what I usually do.
I usually try to keep my relations good with everyone in Empire, and it just makes life easier. Slowly expand, so it doesn't affect your diplomatic relations so dramatically. Also, give small gifts to everyone who is important and you don't want to go to war with every couple of turns, and if you've got no cash, then give them some useless technology (non-military).
But, I kind of do like it the M2TW way. You actually have to convince your allies to help you attack enemies, but you don't have a world war by the 10th turn. And, I guess I choose allies temporarily when it looks like we have a similar objective, and I feel like I get much more help, because it's more like, hey, if we're going to be fighting these Moorish

s, then we might as well do it together. So basically, try to choose allies only when you feel like you have a similar objective and will be fighting on the same front anyway, or choose them so they are less likely to attack.
I'm a huge fan of coming in on a siege with your ally, when you have like 2 spear militia and your ally has a stack besieging the city, and then assaulting the city and auto-resolve... Then you own the city, although your ally did all the work.
