Axis Roll is correct. ANY chance of winning allows the move.
For example, I could send a single SUB against 15 BB’s to attempt to clear a sea zone that my AC would need to pass through in order to recover fighters used in another battle.
Since there is a chance, however minuscule, that the SUB would defeat all 15 battleships, the move IS legal.
And when the SUB dies in the attempt, and the AC cannot move forward to recover the FIGs, the FIGs simply die.
Not a lot of occasions when you would want to try this due to the expense involved. But there are occasions when it may indeed make sense (being able to trash a large number of loaded TRN’s that have no escort, to take an enemy capital, etc.)