The correct answer is - their military involvement in Angola.
It has all started in 1966 with the so called South African Border War that was mainly fought on Namibian soil, but in time the South African forces started hitting on Angolan territory as well and destroyed lots of objects and facilities that were considered of great value for the Angolan economy. The Angolans than got an estimated help of around four billion of modern weapons (for that time) by the USSR and also there were Cuban troops there for help, so they started to turn things around and push the South Africans out of their territory. In South Africa this was used by the government as an anti communist war so that the spread of USSR influence in the region is stopped.
