The United States of America and Mexico have one of the most complex diplomatic relationships in the world. Their interaction touches on every fiber of American life: security, trade, citizenship, pollution, employment, immigration, cross-border issues, cultural engagement, consular protection, financial cooperation, information sharing, etc. The multidimensional character of the interstate relationship is unmatched, yet legal scholars rarely study it. I argue that the complexity of the interaction has birthed a new form of international law that regulates the relationship through the unchecked expansion of letters of intent and memorandums of understanding (inter-institutional agreements). Between 2000 and 2019, U.S. and Mexican authorities signed more than 1,700 agreements. Parties to these agreements include officers from different federal agencies, state and municipal officers, universities, and thinktanks. These agreements are the heart and soul of the legal relationship between the U.S. and Mexico and outlive the cycles of politics. Treaties are no longer the standard for the U.S. and Mexico to collaborate, regulate their interactions, and face common problems. This article sets forth how State behavior is being driven by bureaucracies and their networks, demonstrating how international law operates in practice. Moreover, I argue that at present, regulation of the agreements and obligations between the U.S. and Mexico is inadequate and nearly inexistent, lacking proper congressional supervision by both countries.