Another possibility:
Both sides have to stay for one complete round of combat. Before dice are rolled for the second round, the defender can declare that he will try to retreat some or all of his units. Those units that are retreating do not get to “shoot” in the second round, but can still be taken as casualties.
Example: Germany attacks with 2 tac bombers, 3 tanks, and 8 infantry against a Russian stack of 7 infantry, 1 artillery, 1 tank, and one fighter. The first round of combat kill 5 Russian infantry and 3 German infantry. The Russian decides to leave his 2 remaining infantry as a rear guard, and tries to retreat the rest of his units. On round two, the German rolls for 5 infantry, 2 tac bombers, and 3 tanks, Getting 4 hits. This destroys the 2 Russian infantry left as a rear guard, plus the 1 artillery and 1 tank that tried to retreat with the fighter. The fighter successfully escapes. The Russian rolls only for the 2 infantry left behind, scoring no hits. He saved the fighter, but by trying to save the tank and artillery, he sacrificed the possibility that they would have killed another German infantry or two.