The Mirage III, delivered in the early 1960s, was already too old for the ongoing conflict in South Africa. It didn't have the right moves, speed, or electronic systems to compete with the more advanced MIG-23 in aerial battles. By the 1980s, the South African aviation industry got better at making planes.
This progress let the military upgrade its Mirage fighter planes a lot. Working with the Atlas Aircraft Corporation and the Israel Aerospace Industries (IAI), they created the first Cheetah fighter plane. This change was a big modification of an old fighter into a powerful new one that could compete with the MIG-23 and even the American F-16. Atlas and IAI used existing Mirage III upgrades in the market, like Israel's Nesher and Kfir fighters, and the Mirage 5.
