1. The Soviet Union played a major role in winning World War II. Russia and Ukraine were provinces, not countries. The Soviet Union was much larger than Germany, and Germany was fighting a two-front war against enemies that were larger in terms of population and resources. Note the Russia is half the size of the USSR in terms of population, and has an economy the smaller than several NATO members (US, UK, France, Germany). By threatening the EU and NATO, Russia is in over its head,
2. Had the Allies reached a separate peace, something Stalin was terrified of (those into alternative history would probably focus on acceptance of the very generous terms Hitler offered Britain in 1940, or perhaps if D-Day had failed),the war probably would have ended with the the Third Reich intact but the Soviet Union probably including little more than they had when the war started.
3. Russia is using tactics that worked very well in the 1940s, but the Ukrainians have been trained by NATO in modern warfare (think of major armies launching infantry charges against machine guns in the 20th century – just because the tactic worked in the 19th century when charging muskets). Also, if as reported in the free world, Russian has led its people to believe they are fighting neo-nazis and fascists, and they might expect a problem if they send a large conscript army that realizes it has been lied to. Also, if Russia does general mobilization, the countries on its borders that were formerly under Russian rule (Poland, Finland, Moldova and the Baltic countries) would panic and that panic would spread – and if the “big boys” in NATO start calling up reserves, the Russian situation starts looking very bad. USSR vs. NATO would have been close. Russia vs. NATO/EU would be a disaster for Russia (and perhaps the world).