September 16-17th, 1984: Announcement of majority rebel alliance and affiliated political wings as "South African goerment-in-exile" at 09:00 local time, which is recognised within seconds by the USSR, Cuba, and Angola. (Soviet jets, with Cuban pilots included for optics, are immediately and quietly ordered to African allies) Other African nations follow over the course of the day.
The key event is then 12:20 local time, following hours of propaganda broadcasts primarily aimed at the areas whites are being evacuated from; this is centred around Botha, who condemns the "traitors" for "bringing this great nation to ruin" and calls for "rebellion like the Boers did against previous tyrants". (He is very unhappy it's come to this but this is better than a nuclear war) Sporadic protests, some swiftly turning into riots, spring across the 'abandoned' areas - local rebel militias considered 'unreliable' are told to attack security forces. Operation Deplorable is upended. Faced with orders to use violence, including lethal, against resisters, half the demoralised security forces down arms.
"Exile" forces have been steadily moving small numbers of troops and supplies to the areas where the black population are about to get forced out. At 6:10, the bulk of their forces begin a public march while their advance guard links up with locals and hits strategic targets. The advance guard suffers horrific losses, while the remaining ground-to-air Soviet rockets are used up defending the "frogleap" marching from air attack (which still takes a moderate toll). Radio broadcasts from the "goverment in exile" warn white residents of this battlezone to "evacuate calmly", which gums up roads and prevents regime reinforcements. (Male black residents are told, via different language broadcasts, to assist)
Derby-Smith orders troops to abandon "the traitors" who protested and Botha - along with some loyal white or Jewish communists and two black officers to keep things 'on message' - is rushed into the area, with small number of soldiers, to proclaim this will be a "autonomous soviet" run by locals. (Rebel leadership are divided on what model a future South Africa should be using but in the short term, they want as many white people as possible to think it'll All Be Fine)
In the early hours of the morning, Russian and Cuban-pilot jets enter South African airspace claiming justification under the UN's anti-genocide rules. The Soviets take heavier losses due to their focus on taking out aircrafts that attack rebels; while South Africa technically wins, it means the disputed areas are now almost entirely entire "government in exile" control. At 09:10, the fighting stops as Johannesburg asks for surrender. This is rejected but during the lull, Russian ammunition is being flown in (without it, the rebels would basically collapse at this point). In the regime's remaining territory, over two thousand assumed troublemakers - 90% being black, Asian, or white Jewish - are being executed, while a heavy curfew means only a few hundred at the new border can flee.
This, world leaders know, is the most dangerous point, where Derby-Smith could decide all is lost and go nuclear.