But what if a nuclear warhead gets through and detonates on Ukrainian soil? We should still rely on nuclear deterrence to prevent Putin from using nuclear weapons on a larger scale. It may be possible if difficult to keep a war limited after one nuclear weapon is used. If Putin uses many, he knows that Russia will certainly face nuclear retaliation. But that logic suggests that the United States could turn the table on Putin. If he invades a non-NATO country and uses a nuclear weapon, the gloves should be off with regard to NATO non-nuclear military strikes against Russian military forces in Ukraine and Belarus. The political goal should be—must be—to deny a nuclear aggressor victory, because the global consequences would be dire.

In short, standing up to a Putin who used a nuclear weapon is risky. Living in a world in which there are many nuclear Putins may be even more risky. We should decide which risk is more acceptable, and prepare accordingly.

The immediate implications are clear. The United States should raise the Russian chemical and nuclear weapons contingencies with NATO and propose that NATO begin mobilization for non-nuclear action against Russian forces in Ukraine and Belarus in retaliation. This would primarily mean that NATO air forces, which are already flying patrols with armed weapons, should make plans for air-to-ground strikes against Russian forces. If NATO does not wish to do this, we should discuss what the NATO defense posture should be after Russia conquers Ukraine.