"In 2001, in the wake of the terrorist attacks of September 11th, 'homeland security' went from being a concept discussed among a relatively small cadre of policymakers and strategic thinkers to a broadly discussed issue among policymakers, including those in Congress. Debates over how to implement coordinated homeland security policy led to the passage of the Homeland Security Act of 2002 (P.L. 107-296) and the establishment of the Department of Homeland Security (DHS). Evolution of America's response to terrorist threats has continued under the leadership of different Administrations, Congresses, and in a shifting environment of public opinion. DHS is currently the third-largest department in the federal government, although it does not incorporate all of the homeland security functions at the federal level, even if one constrains the definition of homeland security to the narrow field of prevention and response to domestic acts of terrorism. In policymaking terms, homeland security is a very broad and complex network of interrelated issues. For example, in its executive summary, the Quadrennial Homeland Security Review issued in 2014 delineates the missions of the homeland security enterprise as follows: prevent terrorism and enhance security; secure and manage the borders; enforce and administer immigration laws; safeguard and secure cyberspace; and strengthen national preparedness and resilience."
CRS Report for Congress, R44847
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html