The foreign affairs law of the United States in the decades immediately following the turn of the twentieth century was subject to immense forces of change. Victory in the Spanish-American War at the very end of the prior century – and the consequent acquisition of territories from the Caribbean to the western Pacific – represented a definitive moment in the transformation of the United States from a regional to a global power. The country's dramatic rise as an economic and military force led to an increasing engagement (or entanglement) in world affairs. By the end of World War II the United States was one of only two global superpowers.

These events occurred amid a profoundly expanding faith in international legal cooperation and specifically in the regulatory power of treaties. Before the late nineteenth century, treaties usually served to end wars and settle boundaries, with the prominent exception of early treaties of friendship, commerce, and navigation. The years surrounding the turn of the twentieth century, however, witnessed a significant expansion in the field of operation for international treaties. Large and celebrated peace conferences in 1899 and 1907, for example, proclaimed solemn treaties designed to regulate the law of armed conflict and to settle international disputes. Similar sentiments led to the conclusion of treaties that set the foundation for the later Second and Third Geneva Conventions, as well as the Washington treaties limiting naval armaments and the use of submarines and poison gas.