I'm sorry, I'm not an expert on Africa at all but I did recently go through a phase where I became really interested in Angola for no reason.

The Portuguese arrived in what is now Angola in 1484 and over the next 5 centuries gradually consolidated control over the area. They built towns as trading outposts where they sold manufactured goods from Europe in exchange for slaves sent to Brazil. As you can imagine a lot of stuff happened in between but since your question revolves around 20th century events I will skip ahead.

After WW2, and at the start of the Cold War, there were a class of well educated, westernised native people living mostly in the cities, many of whom were "Mestiços", meaning of mixed racial background. These groups along with rural workers began to form a nascent nationalist movement.

Three distinct strains of nationalism appeared:

1. National Front for the Liberation of Angola (FNLA) - initially a separatist movement made up mostly of members of the Bakongo tribal group who live along the border with Zaire. Many members were migrant workers who travelled back and forth between Angola and Zaire.

2. National Union for the Total Independence of Angola (UNITA) - initially a Maoist guerilla group but they quickly abandoned this, mostly associated with the Ovimbundu ethnic group, based in the remote central region of the country.

3. People's Movement for the Liberation of Angola (MPLA) - formed by a coalition of small political groups including the Angolan Communist Party, associated mostly with the Ambundu ethnic group, attained a broad base of support in rural areas and also amongst the educated natives in the capital, Luanda. They aligned themselves with Marxism-Leninism.

These groups participated in a war of independence against the Portuguese from 1961-1975. They were not particularly successful, probably in part due to the fractious nature of these groups, they did not work together, competed for influence, and had internal divisions as well. The MPLA began to receive assistance from the Soviet Union and Cuba during the war of independence.

This all abruptly ended in 1975 when Portugal's dictatorial government collapsed in the Carnation Revolution. Following the end of Salazar's government the Portugese pulled out of Angola, leaving the nationalist movement essentially victorious. But since they were not a united front, the long running divisions between them quickly erupted into a Civil war.

The Soviet Union and Cuba had already formed a relationship with the left-wing MPLA, whilst UNITA and FNLA had long since abandoned any links with socialism and had been courted by the US and her allies. Over the course of the long civil war the FNLA and UNITA would receive funding, arms and other aid from South Africa, Israel, France and the US, whilst the MPLA were assisted by the Cubans and the Soviet Union. South Africa even deployed troops into Angola, and the Cubans responded from 1975 by intervening militarily themselves. By the end of 1975 there were 25,000 Cuban troops in Angola, and a renewed intervention took place in 1988. They retained a military presence there until 1991.

Throughout the Angolan civil war the Eastern and Western blocs supported their respective proxy forces, although interestingly the FNLA and UNITA also received assistance from the People's Republic of China during a period of tension between the PRC and the USSR.

The Angolan civil war outlasted the Cold war itself, continuing until 2002 when the MPLA effectively won the war. FNLA and UNITA persist as political parties.