two zones of control, other countries' armor units only 5; US armor units, and the two German SS Panzer corps added in 1943, also have 5 strength points rather than the usual 4). British and US infantry have 4 movement points rather than 3, reflecting their higher degree of motorization. Units remain the same throughout the game, as both sides copied one another's technological advances throughout the war. Combat is completely voluntary. Units are always at least doubled on defense (exception: German units in the first winter after their attack on Russia). Terrain, which has no effect on movement, triples units on defense, 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. If the defender is eliminated, victorious attacking units may advance after combat into the hex. Provided at least one attacking unit was armor, any other armor unit which did not attack, but which was adjacent to an attacking unit, may now be placed on the newly gained hex (the "breakthrough hex"). These armored units may then "exploit", i.e. move again, provided they keep to a chain with no more than one empty hex between each unit, and then attack – a key tactic in surrounding enemy units. There is no limit to the number of armor units which may stack on or attack from the original breakthrough hex, provided the overstack is rectified by the end of the turn. Breakthrough against an empty enemy-controlled hex is not allowed, but a key position can be attacked by obtaining an overstacked armored breakthrough against a weakly held hex adjacent to it. An alternative type of combat is "attrition". The attacker totals the land factors in contact with the enemy (air units play no part). The die roll may require the enemy to lose counters (major countries have replacement units