Answer:
The Homeland Security Agency was charged with the responsibility of protecting the US territory from terrorist attacks and responding to natural disasters. This was renamed as Department of Homeland Security after the attacks of September 11, 2001.
While the Department of Defense is responsible for military actions, the Department of Homeland Security works in the civilian sphere to protect the United States inside and outside its borders. Its objective is to prepare, prevent and respond to national emergencies, especially terrorism.