A unit cannot enter a square occupied by an enemy unit, and when directed to do so will attack instead, locking the two units in combat until one is destroyed. An attack usually costs the aggressor one movement point, but results in no actual motion — the surviving unit remains where it was when the combat started. Units with only one-third or two-thirds of a movement point remaining may also attack, but only with that fraction of their normal attack strength. Bombers spend all of their remaining points when they attack, which gives fighters a chance to intercept them.