In 1974, the Portuguese Empire collapsed and Namibia's border with Angola opened up. Nujoma recognised that this paved the way for major changes in the way the war was being fought and over the next two years SWAPO's military campaign shifted its base from Zambia to Angola. The opening of the border enabled thousands of SWAPO supporters to stream out of Namibia to join the movement in exile. Nujoma's son Utoni Nujoma and his two brothers were among those who arrived in Zambia. At the 1977 World Conference Against Apartheid in Lisbon Nujoma underlined the necessity to destroy the colonial system and institutions of South-African racist regime in Namibia in order to build those which will serve the interest of people irrespective of race, religion or origin. He also warned of the danger of installation of neocolonialist marionettes who would superficially change the visible colonial regime while the position of the majority of people would stay the same. In the late 1970s Nujoma led the SWAPO negotiations team between the Western Contact Group (WCG), which consisted of West Germany, Britain, France, the US and Canada, and South Africa on the one hand, and the Frontline States and Nigeria on the other, about proposals that would eventually become United Nations Security Council Resolution 435, passed in September 1978. While agreement on Resolution 435, which embodied the plan for free and fair elections in Namibia, was undoubtedly a diplomatic coup, its implementation became bogged down for another ten years. South African delaying tactics and the decision by American president Ronald Reagan's administration to link a Cuban withdrawal from Angola to Namibian independence frustrated hopes of an immediate settlement. On 19 March 1989, the signing of the cease-fire agreement with South Africa took place, which resulted in the implementation of the UN Security Council Resolution 435.