In 2013, the Obama Administration’s “Nuclear Employment Strategy” guidance announced that all war plans and operations would be “consistent with the fundamental principles of the Law of Armed Conflict” (LOAC). The Trump Administration’s 2018 Nuclear Posture Review repeated this commitment. The literature on nuclear strategy and deterrence in political science however, has either ignored these legal requirements or misunderstood them. The legal literature on nuclear weapons, however, has largely ignored the technical revolution regarding improved accuracy and lower-yield nuclear weapons and the different strategic contexts in which the U.S. might contemplate nuclear use. This paper analyzes how proper application of the Law of Armed Conflict should constrain U.S. nuclear doctrine and war planning and how knowledge of strategic considerations is fundamental to proper legal analysis. We argue that the principle of proportionality can permit “counter-force” targeting— most clearly when such attacks can prevent or significantly reduce the expected damage to U.S. and allied populations with lower foreign collateral damage. We also argue that the legal requirement to take “feasible precautions” to protect non-combatants means the U.S. must use conventional weapons or the lowest yield nuclear weapons possible in any counterforce attack. Finally, we contend that the prohibition against deliberate targeting of civilians has gained the status of customary international law and that the U.S. government should therefore reverse its traditional position and reject the doctrine of “belligerent reprisal” against foreign civilians. This prohibition means that it is illegal for the United States, contrary to what is implied in the 2018 NPR and explicitly maintained by prominent U.S. Air Force lawyers, to either intentionally target civilians in reprisal to a strike against U.S. or allied civilians, or launch attacks against legitimate military targets if the intent to is cause incidental civilian harm.