WASHINGTON—The attacks of Sept. 11, 2001, led policy makers to embark on one of the largest spending binges in federal government history, transforming the private sector, the Washington metropolitan area and Americans’ relationship with their government.

Two cabinet departments—the Defense Department and Department of Homeland Security, which was created after the attacks to consolidate a number of existing agencies—saw huge funding increases as Washington geared up to fight two conventional wars, conduct world-wide operations against small, distributed networks of terrorists and harden the home front against future attacks.