The United States Department of Homeland Security (DHS), commonly known in the United States as "Homeland Security", is a Cabinet department of the U.S. federal government with the responsibility of protecting the territory of the U.S. from terrorist attacks and responding to natural disasters.
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 goal is to prepare for, prevent, and respond to domestic emergencies, particularly terrorism. On March 1, 2003, DHS absorbed the now defunct United States Immigration and Naturalization Service and assumed its duties. In doing so, it divided the enforcement and services functions into two separate and new agencies – U.S. Immigration and Customs Enforcement and U.S. Citizenship and Immigration Services (USCIS). Additionally, the border enforcement functions of the Immigration and Naturalization Service, the U.S. Customs Service, and the Animal and Plant Health Inspection Service (APHIS) were consolidated into a new agency under DHS: U.S. Customs and Border Protection. The United States Federal Protective Service falls under Immigration Customs Enforcement (ICE).
With over 200,000 employees, DHS is the third largest Cabinet department in the U.S. federal government, after the Department of Defense and Department of Veterans Affairs. Homeland security policy is coordinated at the White House by the Homeland Security Council. Other agencies with significant homeland security responsibilities include the Department of Health and Human Services, the Department of Justice, and the Department of Energy.
The creation of DHS constitutes the biggest reorganization of U.S. government in American history and the most substantial reorganization of federal government agencies since the National Security Act of 1947, which placed the different military departments under a secretary of defense and created the National Security Council and Central Intelligence Agency. DHS also constitutes the most diverse merger of federal functions and responsibilities, incorporating 22 government agencies into a single organization.