In the 1980s in Angola, fighting spread outward from the southeast, where most of the fighting had taken place in the 1970s, as the African National Congress (ANC) and SWAPO increased their activity. The South African government responded by sending troops back into Angola, intervening in the war from 1981 to 1987, prompting the Soviet Union to deliver massive amounts of military aid from 1981 to 1986. The USSR gave the Angolan government over US$2 billion in aid in 1984. In 1981, newly elected United States President Ronald Reagan's U.S. assistant secretary of state for African affairs, Chester Crocker, developed a linkage policy, tying Namibian independence to Cuban withdrawal and peace in Angola.