In air combat from counterair or interception each side rolls a die, the larger force adding the number of factors by which his force exceeds the smaller. Italy, France and USSR add a Die Roll Modifier of -1 and minor countries -2. The difference in the rolls is the number of factors lost by the defender - the winner loses half as many; in the event of a tie both sides lose factors equal to a third die roll. Used air units are inverted for the rest of the turn. It is sensible to keep air units out of counterair range, especially inverted units which count for zero in combat. Air superiority is vital in offensives, and the cost of rebuilding air factors is a major drain on BRPs.