There has been no large scale war between major powers since WWII, let alone between the world's leading superpowers.
But deterring a third world war is not one of those things that's subject to moral relativism. You seriously need to spend some time talking with WWII vets, while we still have them with us.
We're seeing the lowest level of conflict in history and you're still unimpressed? Does it kill you that much to admit that MAD is working?