@Driel310 If defenders can’t fire and can’t retreat, then they are all destroyed at the end of the current round of combat. If some defending units are self-sufficient, they can stand and fight while the others are destroyed. And those soon-to-be-dead units can be used as casualties in that round of combat only, as they will all be dead when it’s time to retreat and they cannot.
Page 22: “Each <retreating> unit may move to a different territory, but if no eligible territory is available to retreat to, it is destroyed.”
So those Self-Sufficient units can stay alive into another round of combat if there are enough casualties among the retreaters. The Axis might need only 2 rounds of supply to win in this example. But if there are a couple of Scout Cars and Air Units, Cairo might survive only 1-2 attackers.