Waterboarding and “much worse,” torture, and “tak[ing] out” the family members of terrorists: President Trump endorsed these measures while campaigning for office. After his inauguration, Trump confirmed his view of the effectiveness of torture and has not clearly rejected other measures forbidden by international law. This Article therefore examines whether a President has the power to order or authorize the military to violate international humanitarian law, known as the “law of war.” Rather than assess whether the law of war generally constrains a President as Commander-in-Chief, however, its focus is the extent to which Congress requires the U.S. military to comply with the law of war in its disciplinary code, the Uniform Code of Military Justice (UCMJ). It clarifies how Article 18 of the UCMJ empowers military criminal courts, known as courtsmartial, to try and punish not only conduct denominated a “war crime” by international law but also any other conduct for which the law of war permits punishment by military tribunal. Punishable conduct under Article 18 includes any law of war violation that entails or results in a criminal offense under the UCMJ. Put differently, this Article clarifies why reasonable compliance with the law of war is necessary to justify war measures that are otherwise common crimes such as murder, maiming, and assault, that are defined and made punishable by the UCMJ. This Article then explains why this execution of the law of war in the UCMJ limits a President’s authority as Commander-in-Chief: a President does not possess constitutional power to override congressional regulation of the military, particularly in matters of military discipline. So long as the law of war component of Article 18 remains unchanged, no President may order or authorize war crimes or most other law of war violations that entail or result in a UCMJ offense.