Report to Congress on the Role of the Department of Defense in Supporting Homeland Security [open pdf - 422KB]
"Homeland security is a concerted national effort to prevent terrorist attacks within the U.S., reduce Americas vulnerability to terrorism, minimize damage, and assist in the recovery from attacks. The Department of Defense (DoD) role in homeland security can be summarized as follows: (1) homeland defense, the military protection of United States territory, domestic population, and critical defense infrastructure and assets from external threats and aggression; and (2) civil support, support to U.S. civil authorities for domestic emergencies and for designated law enforcement and other activities. Civil support missions are undertaken by the Department where its involvement is appropriate and where a clear end state for the Departments role is defined. The Department will seek reimbursement for civil support missions when authorized by law."
Defense Threat Reduction Agency: http://www.dtra.mil/