Little has been published on how countries react to the presence of foreign police within their borders. Ethan A. Nadelmann has written a major work on the internationalization of United States criminal law enforcement agencies; its focus, however, is on the behavior and perspectives of U.S. officials engaged in international law enforcement activities. Still, his work has provided interesting clues for my own research on Mexican policy toward drugs in the context of United States-Mexican relations.1 This article will focus on Mexico's experience with the Drug Enforcement Administration (DEA), and in particular, on how and why that United States police agency has become a powerful presence in Mexico and the source of bitter fights between Mexico and the United States over the last fifteen years. DEA'S influence, I argue, has been consequential on two fronts: on the Mexican government and its antidrug policies and on the organization and practice of drug trafficking in Mexico. Leaving aside the critical issue of how, over the last fifteen years, the DEA has modified the relationship between the Mexican government and drug traffickers, in this article I will concentrate on the influence of the agency on Mexican drug policy and on the challenge that DEA agents have come to represent for the Mexican state. That challenge is to the most exclusive, that is, sovereign, of a state's powers, namely, law enforcement. police is not only in charge of legally enforcing a country's laws; it is also the instrument of a central authority. badge and gun of DEA agents represent United States sovereign police power, which does not extend beyond American borders. Many governments have come to accept the occasional practice of hot pursuit across frontiers to arrest fleeing criminals, but very few will tolerate the exercise of police power by a foreign agency. According to Nadelmann, The presence of foreign police may be accepted, but virtually never the legal authority that underpins them in their own country. impossibility of enforcing one country's laws in another polity leads to extremely awkward situations for the DEA in Mexico, which will be described in this article.2