This article assesses American public attitudes toward the just war principles of proportionality, due care and distinction. Consistent with the logic of proportionality, the authors find that Americans are less willing to inflict collateral deaths on foreign civilians when the military advantage of destroying a target is lower. Most Americans also are willing to risk the deaths of American soldiers to avert a larger number of collateral foreign civilian deaths, which accords with the due care principle. Nevertheless, they find that the public's commitments to proportionality and due care are heavily biased in favor of protecting American soldiers and promoting US national security interests. Moreover, they find little evidence that the majority of the public supports the principle of noncombatant immunity, and, contrary to just war doctrine, Americans are more likely to accept collateral deaths of foreign civilians when those civilians are described as politically sympathetic with the adversary.