At least three layers of law both authorize and limit what the U.S. federal government can legally do to counter today's threats. One is the Constitution, a second is statutory law enacted by Congress, and a third is international law. This text examines U.S. national security policy making through the lens of international law, which is composed primarily of treaties ratified by two-thirds of the Senate. The chapters consist of carefully selected excerpts of primary readings to address the question of whether Congress and the President should conform their laws, policies, and actions to the dictates of international law regardless of the nature of the threat. For example, does the President, as the Commander-in-Chief, have a constitutional duty to violate international law if it were necessary to prevent an attack on the United States? In addition, U.S. actions in the name of security have been called into questions by other nations, such as the NSA surveillance practices, targeted killing programs, or the detention of alleged terrorists in Guantanamo Bay. This had led to claims that the U.S. doesn't play by international rules, that it has an "exceptional" role that permits it to engage in conduct that it condemns or criticizes when done by other countries, or that its status as a great power purportedly gives it a license that no other country enjoys. Organized thematically, the chapters present and explain judicial decisions, statutes enacted by Congress, treaties and other such types of domestic and international legal documents in chronological order, underlining how the international, constitutional, and statutory law governing the issue has evolved over time. The readings are relatively short excerpts linked by an introduction to the chapter and by commentaries that lead the student from the specific issue discussed in one excerpt to the next. Each chapter also includes questions to trigger further discussion and promote better understanding of the issue. They cover such key topics as the right to make war, targeted killings, cyber war, covert action, detention of enemy combatants, interrogation of detainees, military commissions, gathering foreign intelligence, and cyber warfare.