what happens if there is a one step unit and +3 fort and Allies use Carpet bombing battle card? Unit is lost but attack continues with odds 6:1+ because battle hex has zero value so Allies probably overruns position except on EX/CR result. Or attack is stopped because Allies cant attack fort alone?
I would say that attack continues. Its true that I cant attack forts alone. I understand it that I cant put there target marker. But if I put initially taget marker there and defending unit evaporates during combat sequence because of combat card, I dont think that attack must be immediatelly stopped.
What do you think?
You got it a bit wrong. The card states that it is played at the START of the Combat Phase, NOT during a battle sequence, like you seem to want to use it.
Start of the Combat Phase means exactly what it says: You thus play it BEFORE even declaring all the combats, not in the commitment step of a battle!
So in your example, you play the card at the start of the Combat Phase, thus eliminating the German unit. You cannot target the hex it occupies as there are no units in the hex anymore and you cannot Target lone Forts (see 22.1 fourth bullet), so the hex cannot be attacked anymore... See it as the Carpet bombing succeeding so well that the terrain is too churned up to make a regular advance possible.
Edit: Making the card be played at the very start of the combat phase is also a "game design ploy" I used so not to have the problematic situation you described above, and more importantly to also represent that historically those valuable strategic bombers would not have been used to pummel a "weak" position held by "only" one step of units. Well, you still can do it, but don't expect it will give you a major "atomic bomb" breakthrough; this did not happen in this theater of operations.