The main function of the Department of Homeland Security is to: protect the United States.
Explanation:
The Department of Homeland Security (DHS) in the United States of America is responsible to protect the US territory against terrorist attacks and help the people during natural disasters.
Defense department takes in charge of military action and protects the US territory inside and outside its borders. The department reacts actively during terrorism.
DHS is the third largest federal department of the US government. The internal security policy for the DHS is postulated by the council members in White House.
