The Portuguese colonies in Africa (Cabo Verde, Guinea-Conakry, Angola and Mozambique) began their struggle for independence in 1960, as French and English colonies were first gaining independence.
However, Portugal was very adamant on maintaining their overseas empire, and maintained an anti-insurgency campaign against independence movements from 1960 until the Carnation revolution in 1974.
In Angola there were 3 main independence movements in this period from 1960-1974. They were the Movimento Popular de Libertacao de Angola (MPLA) which espoused Marxism-Leninism. Frente Nacional de Libertacao de Angola (FNLA) which emerged from a pro-poor farmer political organization, and by 1974 espoused a conservative, christian position. Uniao Nacional para a Independencia Total de Angola (UNITA) which from 1966-79 espoused a Maoist line.
During this period from 1960-1974, these three factions in Angola prioritized the struggle for national independence, and generally sought to minimize infighting.
Additionally, these different factions had international backing. The Soviet Union and the Czech Republic were enthusiastic supporters of the MPLA, providing them with arms and instructors. China provided support to each of the three independence movements at various points in the 1960s, but eventually chose to back the MPLA as the best organized and most effective outfit.
As I explained in this thread, the policy of the United States in the Kennedy administration was initially caught between supporting independence movements and needing Portugal's cooperation within NATO. Succeeding administrations of Johnson and Nixon eventually came down in support of Portugal, while giving minor, token support to FNLA.
Additionally, under Joseph Mobutu (after 1971 Mobutu Sese Seko) the Democratic Republic of Congo ( post-71 called Zaire) adopted an anti-communist orientation and provided arms and support to the FNLA as anticommunist proxies against the MPLA. This was at least in part retribution for MPLA harboring Katangan Gendarmes who had fled after the collapse of Katanga in 1963.
Beyond that, South Africa and Rhodesia generally supported Portuguese rule in Angola and Mozambique, and viewed the prospect of a breakdown in Portuguese rule in those colonies as a threat to the maintenance of White rule in Rhodesia and South Africa.
So, throughout the late 1960s and early 1970s, the MPLA received great amounts of foreign support from superpowers as well as from African leaders like Julius Nyerere of Tanzania and Kenneth Kaunda from Zambia which greatly facilitated their organization and gave them safe havens in Zambia, making them the largest and most effective independence organisation in Angola by 1974.
It was only after the Portuguese pulled out of Angola and Mozambique in 1975 that US foreign policy really came to see Angola as a front in the Cold War chess-match in the Third World. Belatedly, the US and their South African allies came to support the FNLA under Holden Roberto, because of their anticommunist, christian ideological orientation. However, by 1979, the United States had come to regard Holden Roberto and the FNLA as corrupt and ineffective. The United States and South Africa then supported the UNITA movement, which adjusted it's ideological orientation to anti-communism.
I am less familiar with the details of Mozambique, but my understanding is that the broad outlines are similar. The Soviets, Chinese, Tanzania and Zambia all supported FRELIMO (Frente de Libertacao de Mozambique). Again, Tanzania and Zambia allowed training and safe havens for FRELIMO in their territory. The United States, Rhodesia and South Africa supported Portugal
Thus, when a military coup in Lisbon created the Carnation Revolution and radically changed Portuguese policy in favor of ending the colonial wars, it was FRELIMO which was the major anti-colonial force that was prepared to lead a new government.