 These adversaries certainly can work together against us. Isn't it clear that we couldn't defend all of our allies simultaneously? Thus, conflicts that are now discreet in the Taiwan Strait or the Persian Gulf, say, could become theaters of a world war. How about this? Alliances make the most sense only when we and our allies share the same strategic interests. The more specific, the better. The best example is NATO. During the Cold War, it united countries that felt threatened by the Soviet Union.