In the U.S. legal system, the federal government has traditionally been the only rightful arena for the conduct of foreign affairs, especially in the case of national security, military action, international trade, and treaty-making. However, the pervasiveness of globalization and the attendant ease of cross-border interactions, with implications for commerce and terrorism, have brought U.S. states, counties, and municipalities increasingly into the federal government's long-standing province of international relations. For example, states now forge trade relationships with foreign governments through energy and investment contracts that very much resemble treaties. If a foreign sovereign violates any of these contracts or statutes, then civil or criminal action against that sovereign could interfere with the U.S. federal government's diplomatic relations overseas. Consequently, the legal status of states and local governments in the conduct of foreign affairs is unclear and in need of thoughtful analysis and guidance. In Foreign Affairs Federalism, Michael Glennon and Robert Sloane study the constitutional allocation of foreign affairs powers between the federal government and the states. They explain the current law clearly and accessibly, identifying those areas where the law can be confidently ascertained. Where the law cannot be determined, they suggest the most plausible or compelling perspectives on existing doctrine. They also appraise existing doctrine against the background of the diverse and incompatible goals and challenges facing the United States in the twenty-first century.