If this idea is implemented, then there should be some kind of system in place to prevent one country from forming too many military alliances at once. Otherwise, you'll end-up with the sort of "Civ III"-style scenario wherein an incredibly stupid world war would break-out shortly following the invention of Nationalism (if any of you recall, it worked something like this: Civ-A would form a mutual defence pact with Civ-B, only to be attacked by Civ-C, dragging Civ-B into war. A few turns later, Civ-A would sign a war pact with Civ-D against Civ-B, and so on and so on, until it was every civilization for itself).