Ilya’s response to Rick, that the Peace Treaty with Britain’s domestically applicable provisions could have been implemented through the foreign commerce power, seems right to me. But there may be another power that would have justified such legislation.
Peace is the flip side of war. Thus Congress’s power to decide on war also presumably includes the power to make peace, as Madison noted in the 1790s. Just as war does not need to be formally declared, peace can be established without a treaty. There may be international law advantages to a treaty, but peace could be created simply through a the cessation of hostilities, an executive agreement (such as an armistice), and so forth. Thus legislation dealing with the loose ends of a war would be independently justified, to some extent, by the War Power, as the Supreme Court recognized in Woods & Cloyd v. Miller.
Indeed, aside from the treaty with Britain, the Treaty Power would be an incomplete basis for legislating “peace conditions,” as it would potentially be difficult to exercise in cases of debilitatio, the collapse or disintegration of the enemy government.
The Constitution gives the Federal government numerous express powers for directly regulating transborder phenomenon, including war and foreign commerce. The difficulty with the potentially broad uses of the Treaty power today is that they deal with purely internal phenomenon, which are only of general “concern” to foreign countries.