to three times the face value of the ground forces may be added; these factors can be removed as part of an exchange, to allow the survival of ground units, or may be eliminated if that side loses the combat altogether. While the attacker may intercept DAS, a numerically superior defender cannot add extra factors to this air battle; a common house rule was to allow the defender to fly "escorts" to fight off interceptors. 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. Naval warfare Each major power has generic 9-factor fleets: Britain has 10, Italy 5, Germany 4 (2 on the board at the start), France and the USSR 3 each. Four fleets (36 factors) may base at a single port. They may not be voluntarily split up and only whole fleets may be rebuilt (at a cost of 27 BRPs, and with a one-year delay). Fleets may carry supply, may carry ground units for invasions (3 factors to carry each ground factor, i.e. a whole fleet to carry an infantry corps – a beach hex containing a port may not be invaded if a 9-factor fleet is in the hex), may give shore bombardment (3 naval factors count as one ground factor in invasions or attacks on coastal fortresses), may sea transport (carry ground units in the combat phase, 2 factors per ground factor – Germany often transports units into Latvia) or may sea escort (a whole 9-factor fleet to help a single ground unit SR over water). Interception (determined by a die roll) is less likely to succeed over longer distances. Combat is similar to air combat (including similar national DRMs; Germany has a DRM of