In determining its response to the legalization of marijuana in Washington State and Colorado, the federal government need consider not only domestic legal and political concerns, but international relations, as well. The United States is bound by treaty obligations, most notably the Single Convention on Narcotic Drugs; precisely how these treaties compel the federal government to respond to legalization by states (and the role of the supremacy clause) remains unsettled. The United States plays a leading role in international drug-control organizations accorded responsibilities (but no enforcement authority) under the Single Convention, the International Narcotics Control Board, UN Commission on Narcotic Drugs, and UN Office of Drugs and Crime; its authority in these organizations may be undercut if it as seen as violating the treaties, as the INCB President has alleged. The United States also has interests in bilateral and multilateral cooperation on source control and interdiction, including with countries that supply illicit marijuana, although the impact of states’ legalization on drug cartels’ profits and concomitant violence is hotly disputed. Many Latin American leaders who are already disinclined to carry out the United States’ bidding in the global war on drugs have seized on legalization in Washington and Colorado to support their cases; President Calderon of Mexico has said that legalization in the two states limits the United States’ “moral authority” to ask other countries to combat trafficking. Several Latin American countries are considering decriminalization—in Uruguay, legalization—on their own (a debate that Vice President called “legitimate”), now that a precedent has been established. To be sure, some are doing so as a means to wring more military and law-enforcement aid out of the United States. How might the federal government square its apparent international obligations with its reluctance to confront legalizing states? It could simply ignore charges of treaty violations and remain in contravention, seek to remove marijuana from the treaties’ schedules, or withdraw from the controlling treaties.  This paper will examine the constraints that international relations and law place on the federal response to state-level legalization, assess consequences for US foreign policy, and explore possible actions by other countries.