The United States Department of Homeland Security (DHS) is a cabinet department of the United States federal government, created in response to the September 11 attacks, and with the primary responsibilities of protecting the territory of the United States and protectorates from and responding to terrorist attacks, man-made accidents, and natural disasters. In fiscal year 2011 it was allocated a budget of $98.8 billion and spent, net, $66.4 billion.

Whereas the Department of Defense is charged with military actions abroad, the Department of Homeland Security works in the civilian sphere to protect the United States within, at, and outside its borders. Its stated goal is to prepare for, prevent, and respond to domestic emergencies, particularly terrorism.[5] On March 1, 2003, DHS absorbed the Immigration and Naturalization Service and assumed its duties. In doing so, it divided the enforcement and services functions into two separate and new agencies: Immigration and Customs Enforcement and Citizenship and Immigration Services. The investigative divisions and intelligence gathering units of the INS and Customs Service were merged together forming Homeland Security Investigations. Additionally, the border enforcement functions of the INS, including the U.S. Border Patrol, the U.S. Customs Service, and the Animal and Plant Health Inspection Service were consolidated into a new agency under DHS: U.S. Customs and Border Protection. The Federal Protective Service falls under the National Protection and Programs Directorate.