I'm guessing you're talking about World War II?
In Great Britain, the short answer is that it's an island. It hadn't been somewhat successfully attacked since 1263. I'm talking about by foot, not by air. A lot of this gave the Brits a sense of patriotism and morale. (It's also why the event of Dunkirk was so important). It also deterred Hitler during the Battle for Britain. Here's a good example, the Germans would keep losing aircraft with each bombing raid over Great Britain, and though the Brits would lose planes it would be less compared to their enemies. This is because Great Britain was able to salvage materials from their lost aircraft, unlike the Germans. I believe Hitler got bored and began focusing his attention from Great Britain to the USSR about two weeks before the Brits would be forced to surrender.
The Soviet Union was a different story. Although the country was the largest in the world, they were no match for Germany's brutal army in the beginning. Hitler began his betrayal of Russia in June and planned to have his troops housed in major cities during the cold winters. However, because of Mussolini's failure to capture Greece, some troops were spared down south making the trek into Soviet territory slower. German battalions weren't able to reach their goal by the beginning of the Russian winter. And just like Napoleon, Hitler had made a grievous error. This is when the Russians began turning the tables. Since the battlegrounds were in more familiar places, the Soviets were in their natural setting. The Germans, on the other hand, were ill-equipped. They still were in their summer uniforms (a weird green-grey) and the Russians were in white to blend in with the snow. Stalin also used the Scortched-Earth strategy where the Russian troops burned down farms and other things that could aid the Germans. German supply roots were cut off as well.
Germany prepared for a world war long before everyone else did. In addition to building up his army, Hitler expanded railways and roads all across the country for an easy access to move thousands of troops from one side of the country to another. Of course, at the time he said it was for civilians so the League of Nations didn't bat an eye. Joining the army was largely encouraged through the Hitler Youth (Hitler- Jugend). Basically, it entailed that young boys will be battle ready by the time they were of age. Young girls were promoted to have many children and keep up with housekeeping in the Bund Deutscher Mädel. Everything anyone did was to better Germany.
So, yeah! I'll add some stuff if something comes to mind later. Cheers!
