Two-term Arizona governor Janet Napolitano became the third secretary of Homeland Security last January 21, replacing Michael Chertoff as head of the massive agency. With more than 200,000 employees, including the Transportation Security Administration, the DHS is the third largest of the Cabinet departments. Chertoff, who has known Napolitano since both were prosecutors, suggested that her biggest challenge would be confronting “deeply embedded special interests” from industry, labor and elsewhere.