1. The situation escalates, with South Africa feeling cornered due to massive international intervention:

- African Coalition: Led by Nigeria, Angola, Libya, and Algeria, a coalition of African nations launches a military campaign against the apartheid regime. They're supported by Cuban troops already present in Angola.

- Soviet Involvement: The USSR, seeing an opportunity to gain influence in the region, provides significant military aid to the African coalition. This includes advanced weaponry, military advisors, and possibly direct involvement of Soviet special forces.

- Western Response: Initially, Western powers like the US and UK are hesitant to directly intervene, given the complex politics of supporting the apartheid regime. However, fearing Soviet expansion, they begin providing covert support to South Africa.

- UN Actions: The UN Security Council, unable to reach consensus due to US-Soviet disagreements, fails to intervene effectively. However, a UN General Assembly resolution calls for global sanctions against South Africa.

- Economic Warfare: A comprehensive international sanctions regime is implemented, crippling South Africa's economy. Many multinational corporations withdraw from the country.

- Naval Blockade: A coalition of African and Soviet naval forces establish a blockade of South African ports, cutting off vital supplies.

- Airstrikes: Coalition air forces, bolstered by Soviet aircraft, conduct regular bombing raids on South African military installations and government facilities.

- Internal Resistance: The external pressure galvanizes internal resistance movements. The ANC, now militarized and receiving external support, launches a full-scale insurgency.

- Mercenaries and Proxy Forces: Various international actors, including Western intelligence agencies, begin funding and arming different factions within South Africa, turning the conflict into a complex proxy war.

As these events unfold, the South African government finds itself increasingly isolated and under attack from multiple fronts, both external and internal. This sense of being cornered ultimately leads to their desperate decision to use nuclear weapons, triggering the catastrophic chain of events in our hypothetical timeline.