Under dos Santos's leadership, Angolan troops crossed the border into Namibia for the first time on 31 October, going into Kavango. The next day, dos Santos signed a non-aggression pact with Zambia and Zaire. In the 1980s, fighting spread outward from southeastern Angola, where most of the fighting had taken place in the 1970s, as the National Congolese Army (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 MPLA more than 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.