I almost think defensive pacts could be better if they worked the other way around. If there are people who think your expansion is a threat nearby, and they see you attacking in another direction, they may offer to join in to defend them and limit your expansion. Not quite a permanent alliance, but something to worry you. The advantage of this is that it makes the pact contingent on the nature of the war being fought- so people in India aren't going to automatically get involved in a war in the Balkans, just because the defensive pact is not location-tied