This publication, "Homeland Defense (Joint Publication 3-27)," provides doctrine for the defense of the US homeland across the range of military operations. It provides information on command and control, interagency and multinational coordination, and operations required to defeat external threats to, and aggression against, the homeland. Defense of the homeland is the Department of Defense's (DOD's) highest priority with the goal to defeat threats at a safe distance from the homeland. Homeland defense (HD) is the protection of US sovereignty, territory, domestic population, and critical defense infrastructure against external threats and aggression, or other threats as directed by the President. DOD is responsible for the HD mission, and therefore leads the HD response, with other departments and agencies in support of DOD efforts. In today's complex threat environment, our approach to HD must address all aspects of the operational environment. Externally, the United States has sought to shape the international environment through the application of diplomatic, economic, military, and informational means. The homeland is confronted by a variety of interrelated threats that demand coordinated procedures and synchronized efforts among US Government (USG) departments and agencies charged with law enforcement and national defense. These threats include any transnational activity including international terrorism, narcotics trafficking, the proliferation of weapons of mass destruction and the delivery systems for such weapons, and organized crime that threatens the national security of the United States. These threats also include extremists and opportunists who enter into relationships of convenience that exploit the capabilities of the other and cloud the distinction between crime and terror. Evidence suggests that terrorist organizations have grown more extreme in their objectives and actions and are less concerned that attacks on innocent civilians or public infrastructure will undermine support for their causes. The Strategy for Homeland Defense and Civil Support calls for securing the United States from attack through an active, layered defense in depth. This active, layered defense seamlessly integrates US capabilities in the forward regions of the world, in the geographic approaches to US territory, and within the US homeland. This publication has been prepared under the direction of the Chairman of the Joint Chiefs of Staff. It sets forth joint doctrine to govern the activities and performance of the Armed Forces of the United States in operations and provides the doctrinal basis for interagency coordination for defense of the homeland. It provides military guidance for the exercise of authority by combatant commanders and other joint force commanders (JFCs) and prescribes joint doctrine for operations and training. It provides military guidance for use by the Armed Forces in preparing their appropriate plans. It is not the intent of this publication to restrict the authority of the JFC from organizing the force and executing the mission in a manner the JFC deems most appropriate to ensure unity of effort in the accomplishment of the overall objective. Joint doctrine established in this publication applies to the commanders of combatant commands, sub unified commands, joint task forces, subordinate components of these commands, and the Services.