@Flashman:
France is faced with a German stack of 12 infantry.
Compare
Case A: France attacks with 18 infantry (54 IPCs).
Case B: France attacks with 6 infantry & 6 tanks (54 IPCs).
The upshot is that in Case A, France eventually wins and is left with 10 infantry (30 IPCs).
In case B, France wins and is left with 2 infantry & 6 tanks (39 IPCs). It could of course choose to take tanks as casualties, but this rather defeats the object of building tank stacks.
…and case C, France attacks with 8 inf, 3 art, 2 tanks and 1 aircraft (54 IPC)
Air supremacy make all 3 art attack on 4 or less, and the art boost up the 2 tanks and 3 inf to attack at 3 or less, and the rest of the inf attack at 2 or less.
This combo will likely get 6 to 7 hits.
The 12 defending inf will get 6 hits, but our 2 tanks will soak up two of the hits, so we only lose 4 inf.
Now the defender has lost units of 21 IPC’s value, while we only lost units of 12 IPC’s value.
And compared to case A, we have saved units of 24 IPC’s value, and to case B we have saved 3 IPC’s.
But when case C defend, they will make at least 5 or more hits on the enemy, while case B will make 2 hits.