This article considers the post–September 11 challenges faced by the U.S. national security machinery and analyzes the relationship between the new threat environment, the United States’s role in the world, and decision–making structure. Homeland security is defined as a subset of national security. The threat of foreign terrorist organizations acting on U.S. soil should be seen in the political context of the pursuit of U.S. national interests in an often anarchic world. Two models for homeland security organizational structures are considered: a departmental model and an interagency model. The interagency model, embodied in the Homeland Security Council, is a better fit given the nature of the threat, the crucial need for coordination, and the realities of governmental decision making. The organizational challenges that may complicate the government’s preparations, decisions, and implementation of a major homeland defense mission stem from three rivalries: executive–legislative, cabinet–staff, and Homeland Security Council–National Security Council.