A distinction is made between cooperation among national police forces on the one hand (including harmonization of their organizational structures, their operational methods, their technical equipment, and their legal authority) and their integration in a single transnational system. Interpol, founded in 1923 in response to the problems following the First World War and the Russian revolution, is an example of the former. Napoleon's police force in the 19th Century and the Nazi S.S. in the 20th Century are examples of single, transnational police forces. In this century, especially since the 1960's, the United States has been a leading advocate and actor of cooperative police work among nations. Nearly 250 members of the DEA (Drug Enforcement Administration) are active in 40 countries, and 30 FBI members perform duties in 10 countries. Their activity is principally to apprehend drug traffickers; they depend on the cooperation of local police and have in turn been of assistance to local police forces fighting drug trafficking. American tactics, such as "buy and bust," have now entered police practices of other countries. Efforts to bring more cooperation among police in Europe now focus on the ratification of the Treaty of Maastricht and the Convention of Schengen; the latter specifically deals with police cooperation and the creation of Europol, an intergovernmental police force. The author concludes that while police cooperation in Western Europe will no doubt grow, struggles in some Eastern European countries form obstacles to their participation except as members of Interpol and perhaps as part of a United Nations police force.