Was talking to my uncle about the world wars, and he mentioned that he thought the russia had been planning to invade germany before germany struck first. Also that part of the reason the germans were so successful early on was because the russians had moved supplies forward towards the german border and they were captured. He's a history buff, so I'd be surprised if he said this without reading it from something fairly reliable.
It seems pretty easy to imagine anyways. Spreading communism, bad feelings from WWI (?), and expansionist policy all seem like fairly good reasons to be gearing for an invasion.
I have read that both Stalin and Hitler were expecting an attack by the other. The Molotov-Ribbentrop pact meant they would not have war between themselves but deep down both leaders knew this wouldn't last. But as history played through the Germans attacked first, devastating the Soviet defences