The fatal flaw of the warm and progressive-sounding NFU proposal is that it tells would-be aggressors that they do not have to fear U.S. nuclear retaliation as long as they attack us or our allies with advanced conventional, chemical, and/or biological weapons. They would risk U.S. nuclear retaliation only if they attack with nuclear weapons.

Numerous historical case studies demonstrate without a doubt that some aggressors look for such openings to undertake military moves they deem critical. They do not need to see a risk-free path to pursue aggression, only a path that allows them some vision of success, however improbable that vision may seem to others. The U.S. nuclear deterrent helps to shut down the possibility that would-be aggressors will contemplate such paths.

A U.S. NFU policy would be particularly dangerous at a time when Russia and China may be armed with chemical and biological weapons and are pursuing expansionist policies in Europe and Asia, respectively. Russia is by far the strongest military power in Europe. It has moved repeatedly against neighboring states since 2008, forcibly changing established borders in Europe for the first time since World War II and issuing explicit nuclear first-use threats in the process. Only several months ago, Russia reportedly rehearsed the invasion of Norway, Finland, Sweden, and Denmark in a military exercise involving 33,000 troops. In Asia, China is the strongest military power and is expanding its reach against U.S. allies, with tactics that include building and militarizing islands in the South China Sea. At a time when key U.S. allies face unprecedented threats from powerful neighbors, the U.S. should not reduce the calculation of risks that Russia and China must confront in their respective expansionist drives by adopting an NFU policy. Indeed, this is a breathtaking understatement in a world in which aggressors still exist, as do advanced conventional, chemical, and possibly biological weapons, and in which another world war using “only” such modern non-nuclear weapons could cause death levels far beyond the 80 to 100 million lost in World Wars I and II.