The Nazi invasion of the Soviet Union in 1941 can be best understood by remembering one quotation:

We have only to kick in the door and the whole rotten structure will come crashing down.

The Nazis were extremely confident of a quick and easy victory against the USSR in 1941, and in the first few months of the war, they didn't have much reason to re-evaluate. In the first two months of the war, the Nazis took over a frankly gigantic swath of territory, including what's now Estonia, Latvia, Lithuania and a big chunk of both Ukraine and Belarus. The Soviets were falling back - and they'd continue to do so until December 1941, by which point the Nazis were at the gates of Leningrad and Moscow, and were also within striking distance of the Caucasus - which is to say, within striking distance of Soviet oil supplies.

If the Nazis had subsequently taken Moscow, then it's possible that the Soviet Union would have come crashing down. However, the Nazis were behind schedule and the Russian winter hit.

The mean daily temperature in Moscow in November is -1.2 °C and -7.6 °C in December.[1] From November to March, the mean daily temperature in Moscow does not climb above freezing - and that's just for an average winter. The winter of 1941-1942 was actually the coldest of the twentieth century.

If you want to have your soldiers in anything approximating fighting condition in that climate, you need to get them warm boots, warm gloves, warm hats and warm jackets. If you want them to actually be able to fight, there are extra supplies required for weapons maintenance in the cold that aren't required in temperate weather. Motorized vehicles are harder to maintain in the cold - just ask anyone who's had difficulty getting their car engine to turn over in the middle of a Toledo winter.[2]

Because the Nazis had expected a quick war, they had brought none of these supplies with them. Of course, Nazi Germany was an industrialized society, one where the military-first approach to the economy had led to shortages of wood and butter for civilians in the 1930's and that could therefore create these supplies. However, this ended up being rendered somewhat moot by two issues. First, the Nazis had 3.6 million troops in Russia that required the winter supplies. That's a lot of winter supplies to transport.

The second factor was that Operation Barbarossa had been far too successful for a military operation that had not achieved its primary objective of knocking the Soviets out. The result of this was that the Nazis had troops all over 1.3 million square kilometers[3] of Soviet territory - territory populated by people who, with very, very good reason, hated the Nazis and weren't much for cooperating with them.[4] And the people who most needed the supplies were furthest from where the supplies were created.

This would have made it difficult for the Nazis to supply themselves even under the best of circumstances - and the Nazis weren't experiencing those. Many of the roads the Nazis had to use to invade Russia were dirt roads. Dirt roads do cause more damage to automobiles and other mechanized vehicles than do paved ones, but that actually turned out to be of much less importance than Russian weather did. Specifically, it rains heavily during autumn in Russia. This period is known as the "rasputitsa," and during this time, if your road isn't paved, it's essentially a river of mud. It's hard enough to march men through that sort of terrain - getting vehicles laden with supplies through it is damn near impossible. To top it off, the Nazis didn't have anywhere near enough trucks to supply the invasion, so they were heavily reliant on horse-drawn wagons for logistic support.

The result was that, while the Nazis did manage to make it to the outskirts of Moscow, they only did so in December - and few people in the Third Reich had planned on the war running that long. Those winter supplies needed to fight? They didn't arrive in anywhere near the volume required, and many Wehrmacht soldiers died of exposure as a result.

The Soviets didn't have this p