It wasn't that they wanted to conquer Europe, per se. Their primary concern was the security of the Soviet Union. They were every bit as concerned about the Western threat to their way of life as we were about theirs. However, Soviet doctrine has two basic tenets: Offence is the best form of defense, and don't do things by halves. If the USSR felt itself to be under threat, then they would attack and remove the threat. Not just contain it, so that it may rise and strike again later, but to get rid of it entirely, and replace it with a fraternal (socialist) government which won't attack again in the future. Fortunately, the Soviets never found it to be in their best interest to do so, despite the various surrogate Cold War fights over the second half of the 20th Century which could have spiraled out of control without restraint on both sides.