Historians often say that Hitler was defeated on the Eastern Front in World War II by three great Russian generals: General Mud, General Winter, and General Distance. I think that's pretty accurate.
Being such a large country, there is no way to conquer the Soviet Union in World War II with conventional forces because supply lines would inevitably become overextended and vulnerable. That was just one reason why Hitler's invasion of Russia was so foolish militarily.
The rains that hit parts of Russia (like Belarus and the Ukraine) in the spring and fall turned the primitive roads into oceans of mud, which paralyzed the trucks and tanks of the Wehrmacht for weeks at a time. In September of 1941, for example, it stalled Hitler's army for two crucial weeks on the drive towards Moscow. His army would come within 20 miles of the capital and no closer.
The topography was fairly easy, with no major mountain ranges in most of the country, but there was a number of river systems which made for natural defensive lines, and the wide open steppes were easy to drive on, but very difficult to defend, as Hitler's retreating army would later discover.