DHS is responsible for protecting the United States from terrorism, securing the nation's borders, and coordinating the federal response to natural disasters. Formed by merging 22 previously separate federal agencies, it is the largest cabinet reorganization since the Defense Department's creation in 1947.

The department's components include Customs and Border Protection, which secures the border and staffs official ports of entry, Immigration and Customs Enforcement, which handles interior immigration enforcement, the Transportation Security Administration, which screens airline passengers, and the Federal Emergency Management Agency, which coordinates disaster response.

DHS also houses the Secret Service, which protects the president and other senior officials, the Coast Guard, which normally operates under DHS but shifts to Defense Department control during wartime, and the Cybersecurity and Infrastructure Security Agency, which defends federal networks and critical infrastructure against cyberattacks.