According to this legislation, what is the main function
of this department?
There is established a Department of Homeland Security,
as an executive department of the United States.... The
primary mission of the Department is to (a) prevent
terrorist attacks within the United States; ...(c) minimize
the damage, and assist in the recovery, from terrorist
attacks that do occur within the United States; (d) carry
out all functions of entities (separate things] transferred
to the Department....
recovering from terrorist attacks
managing new executive departments
planning for natural emergencies
protecting the United States

The United States Department of Homeland Security (DHS) is a US government department whose responsibility it is to protect the US territory against terrorist attacks and to act in the event of natural disasters.

While the Department of Defense is in charge of military action abroad, the Department of Homeland Security works in the civilian sphere to protect the US territory inside and outside its borders. Its purpose is to prepare, prevent and respond to domestic emergencies, particularly terrorism.

With more than 200,000 employees, DHS is the third largest federal department of the US government, behind only the Department of Defense and Department of Veterans Affairs. The internal security policy is coordinated by the White House by the Homeland Security Council. Other agencies with significant internal security responsibilities include the Department of Health and Human Services, the Department of Justice, and the Department of Energy.