In 2013, the U.S. government announced that its nuclear war plans would be “consistent with the fundamental principles of the Law of Armed Conflict” and would “apply the principles of distinction and proportionality and seek to minimize collateral damage to civilian populations and civilian objects.” If properly applied, these legal principles can have a profound impact on U.S. nuclear doctrine. The prohibition against targeting civilians means that “countervalue” targeting and “minimum deterrence” strategies are illegal. The principle of distinction and the impermissibility of reprisal against civilians make it illegal for the United States, contrary to what is implied in the 2018 Nuclear Posture Review, to intentionally target civilians even in reprisal for a strike against U.S. or allied civilians. The principle of proportionality permits some, but not all, potential U.S. counterforce nuclear attacks against military targets. The precautionary principle means that the United States must use conventional weapons or the lowest-yield nuclear weapons that would be effective against legitimate military targets. The law of armed conflict also restricts targeting of an enemy’s leadership to officials in the military chain of command or directly participating in hostilities, meaning that broad targeting to destroy an enemy’s entire political leadership is unlawful.