You mentioned Cuba, which was in fact heavily involved with it and did in fact intervene both military and non-militarily. First of all there's the Angolan Civil War which played a large role. In 1975, South Africa (urged by Washington) invaded Angola to prevent FAPLA from taking over which was on the verge of winning the civil war. Cuba decided to intervene and sent a force of 36000 soldiers which successfully pushed back the South Africans.

Beside the military victory, this also had a pretty big moral effect in inspiring ANC and similar groups. It was the first time the South African army was defeated, and by a non-white army at that. According to Nelson Mandela, this was also "the first time that a country had come from another continent not to take something away, but to help Africans to achieve their freedom", which he wrote on the event while he was in jail. The major black South African newspaper "The Word" wrote this on the event: "Black Africa is riding the crest of a wave generated by the Cuban victory in Angola. Black Africa is tasting the heady wine of the possibility of achieving total liberation"

Years later, in 1991, during his visit to Havana Mandela personally thanked Cuba for their support and spoke of the events in Angola:

The Cuban victory destroyed the myth of the invincibility of the white oppressor and inspired the fighting masses of South Africa. Cuito Cuanavale is the turning point in the liberation of our continent and of my people from the scourge of apartheid

[...]

I should mention that when we wanted to take up arms, we approached numerous Western governments in search of help and we could only talk with the lowest level officials. When we visited Cuba we were received by the highest authorities who immediately offered anything we wanted and needed.

[...]

The decisive defeat in Cuito Cuanavale changed the status of forces in the region and reduced considerably the capacity of the Pretoria regime to destabilize its neighbors … within the country

It is in this context that we value our friendship with Cuba very, very much. When you, Comrade Fidel, yesterday said that our cause is your cause, I know that that sentiment came from the bottom of your heart and that that is the feeling of all of the people of revolutionary Cuba.

After this event (pushing back the South African invasion), Cubans remained in Angola to protect it in case SA attempted another invasion. And during this time, Cuba was heavily involved in both training ANC and supplying it, which resulted in strong relations developing between the two.

In addition, the USSR was also quite heavily involved, not as much militarily as Cuba, but they invested a lot of financial and military aid in Angola and ANC.

Source: Piero Gleijeses, Visions of Freedom: Havana, Washington, Pretoria, and the Struggle for Southern Africa, 1976-1991