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> How much the luck contributes to the outcome of those games and how much the tactic/strategy the players employ?
On Battle for Moscow, as is typical for this kind of wargame, luck is only a tiny factor. Within individual battles, terrain and the strength ratio of attacker define which column of the Combat Results. As you can see, the defender only has a chance to win on a 1:1 or 2:1 ratio; luck just decides casualties, and even that is influenced by the ratio. This adds up with terrain, initial force distribution and number of hexes and units to dillute the importance of die rolls.
On Eastern Front 2, luck is more important due to a different combat system and a higher level of abstraction, but not to the relevance it holds in Risk.