(normally Britain). Germany's SR limit (9), although large, is not enough to cope with war on three fronts from 1944 – rebuilt Hungarian units can take a year to walk back to the Eastern Front – and can be a key factor in her collapse in the late game. Supply may not be traced through zones of control, even if the hexes in question are occupied by friendly units. Unsupplied units cannot move, and are eliminated after the construction phase, and cannot be rebuilt in the same turn in which they are lost (unless the owning player deliberately loses them in poor-odds attacks). Air warfare Paratroops do not count for stacking, and negate the effect of rivers, but are lost permanently if not adjacent to a friendly unit when eliminated. They may also drop during armored exploitation. For the first half of the game the German FallSchirmJaeger Corps is the only paratroop unit on the map. Britain and Italy each add a paratroop corps to their forcepools in 1942 (the USSR adds two slightly weaker ones); the USA also has a paratroop corps in her forcepool. Up to five air factors may base at a city, or at an airbase counter (each major power has three of these, one of which may be placed each turn – once placed it can only be moved by SR or relocated to the country's capital if out of supply). Initially Germany has 6 air units and Italy 2 (i.e. 40 factors), against three British and two French (i.e. 25 factors). By the end of the war 5 German air units (i.e. 25 air factors, assuming some have been transferred to fight Allied bombers) might be facing 5 US, 4 British and 3 Soviet (i.e. 60 air factors). Air units may be voluntarily split up, and single factors may be rebuilt provided they are combined by SR to form a complete air unit. Air units may "stage" (move between bases up to twice their combat range – during staging or SR they may fly across water up to their combat range) and still be used for combat. Air units may Counterair (attack enemy air at their bases – British air factors in Malta may decline combat but must be inverted if they do so), give Ground Support to attackers (including exploiting armor) or Defensive Air Support. For Ground Support or DAS air factors up