Combat is completely voluntary. Units are always at least doubled on defence (exception: German units in the first winter after their attack on Russia). Terrain, which has no effect on movement, triples units on defence, while fortresses (Maginot Line, Westwall, Gibraltar, Malta, Leningrad and Sevastopol) quadruple them. Each combat may result in the complete elimination of either side, or an exchange, or a compulsory counterattack, either at face value (i.e. ignoring terrain effects) or at odds dictated by the table. A 2:1 attack is almost certain to succeed (albeit with some risk of an exchange), with only a 1 in 36 chance of an "attacker eliminated" - the dreaded roll of 4, followed by 6. An attack at odds of 3:1 or greater is guaranteed to succeed, the only uncertainty being whether or not the defender will be eliminated outright or inflict some exchange losses.