It is said, “Don’t attack Russia in winters” because both Napoleon and Hitler were defeated when they tried to invade and conquer Russia. The average temperature around Moscow and its surrounding area is typically -10 degrees Celsius. Both Napoleon and Hitler underestimated the severity of the Russian winter and the difficulties of re-supplying their armies over difficult terrain, especially in winter. Hitler also did not hold the Russians in high regard because of his racism.
And, Hitler was such a know-it-all, he didn’t believe in weather forecasts. Of course the weather in Russia during the winter was not the only reason for their defeat. Both Napoleon and Hitler faced fierce resistance from the Russian soldiers and people