The Department of Homeland Security is a Cabinet department of the United States 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 stated goal is to prepare for, prevent, and respond to domestic emergencies, particularly terrorism.
During peacetime, the U.S. Coast Guard is under the supervision of Homeland Security, whereas during war, it functions under the Department of the Navy.