What led to the start of the CIA and what was its purpose?

The 1947 National Security Act established the CIA, affording it "no police or law enforcement functions, either at home or abroad". One year later, this mandate was expanded to include "sabotage, anti-sabotage, demolition and evacuation measures...subversion [and] assistance to underground resistance movements, guerrillas and refugee liberation movements, and support of indigenous anti-communist elements in threatened countries of the free world.