The Office of Homeland Security was organized in the days following the September 11, 2001, terrorist attacks. President George W. Bush (1946-) chose Pennsylvania governor Tom Ridge (1945-) as the first Office of Homeland Security advisor. Ridge was sworn in on October 8, 2001. The office was elevated to the department level on November 25, 2002, when President Bush signed into law the Homeland Security Act, creating the Department of Homeland Security (DHS) and making Ridge a cabinet-level administrator.

The DHS consolidated several existing agencies and pledged to carry out new initiatives to, the extent possible, protect the nation from further attacks. Agencies and sub-departments within the DHS’s purview eventually included the Transportation Security Administration, Customs and Border Protection, Immigration and Customs Enforcement, the Federal Emergency Management Agency (FEMA), Information Analysis and Infrastructure Protection offices, U.S. Citizenship and Information Services (formerly the Immigration and Naturalization Service, or INS), an office for Civil Rights and Civil Liberties, the U.S. Coast Guard, and the U.S. Secret Service.

On February 15, 2005, Ridge was succeeded by Michael Chertoff (1953-), a former U.S. Circuit Court judge. Chertoff had also worked as an assistant attorney general; in that position, he helped trace the 9/11 terrorist attacks to the al Qaeda network and worked to increase information sharing within the Federal Bureau of Investigation (FBI) and with state and local officials.