The ability to "nuke an entire continent" isn't saber rattling, it's holding a dead man's switch. All major powers have been holding them for decades, and piling more nukes on doesn't make any difference at this point. All mutually assured destruction does is prevent attacks in the physical domain from happening*. So instead the fights continue in the political, economic, and digital domains. If anyone makes a physical attack in response to a nonphysical one, they have already lost.

* against other major powers, of course. Major powers themselves continue to test what they can get away with. Russia moving into Crimea. China saying "What? These guys have always been part of China. Do something, I dare you."