Let's say that Gerald Ford wins the 1976 election. The US government quitely watches the growing war in Namibia and Angola, and begins sending limited clandestine aid to South Africa via the CIA as the Soviet and Cuban involvement increases. In addition, let's say that Angola allows the Soviets to maintain bases on Angolan territory, making the situation look much bleaker for anti-communist forces.
When PW Botha becomes Prime Minister of South Africa in 1978, talks begin with the US government on increased involvement in the Namibian situation. Botha needs more US help to prevent the Angolan communists from exporting their revolution to Namibia via SWAPO. However, the US is hesitant to provide aid due to apartheid. So, a deal is struck. Botha will begin to downsize apartheid policies and liberalize to a degree as well as agree to the principle of an independent Namibia in exchange for a direct US presence in Namibia.
The US begins by sending military advisors, and slowly increases its presence as Angola builds up support for SWAPO. Eventually, the US builds up some 10,000 troops in Namibia under the guise of peacekeepers providing a stable environment for the creation of a democratic government in Namibia. This US presence was at its peak in 1987, as the Cubans and the South Africans duelled in Angola. While US forces did not officially participate in cross-border operations, US forces did engage Angolan and Cuban forces which crossed the Namibian frontier several times. Most of the US military operations were focused on SWAPO and on providing aid to anti-communist guerillas.
With a nailed down deal on Namibian independence reached by 1989 with all parties involved agreeing, the US began to draw down its forces in Namibia, and by 1994, the US presence was gone.