A fixed defense is a lost cause. The best defense is actually to fall back and give up ground if you have it. The German losses in the East were largely because of ill-fated arrogant attempts to hold ground. Air power means everything. Since WWII, the victors have always had air supremacy.
Many historians now consider both wars as part of the same conflict.
It's best to end a war as friends. The Nazis were true believers, they really believed that they had to stop an evil conspiracy by the Jews and usher in a golden new age for humanity. It's important to note that 2 democracies have never fought. Countries with a high respect for basic human rights are rarely aggressors. The communists didn't care how many losses they incurred.