Source: here.
Gauge Transformation
As a reader pointed out, you don’t need to to have a flypaper memory to be a physicist. There you can go a long way with a few basic facts and a long chain of mathematical reasoning. You can see that the equation of state is going to go soft when things turn relativistic, and suddenly the star collapses. Whee!
But many subjects are not like that. Understanding, to the extent that it is possible at all, requires mastery of many facts. Consider Barbarossa, the invasion of the Soviet Union. More than three million Germans invaded, with hundreds of thousands of motor vehicles and even more horses. The Germans had the initiative and were tactically superior: the Russians had greater numbers and resources, if they could survive long enough to bring them into play. The Germans had to win quickly, if they were to win at all.
Modern armies require enormous amounts of supply, mostly gas and ammo. In those days, supply mainly meant railroads. In Germany, and in most of Europe, the separation between the rails, the rail gauge, was 1,435 mm (4ft 8 1/2 in). So when the Germans invaded France, they could immediately make use of the French railnet. In Russia, the Germans faced a problem: the gauge was different, 1528 mm (5 ft). German locomotives could not use those tracks until they had been converted. As Pravda used to say, this was no coincidence: it is thought that the Czarist government made this choice for defensive reasons.
It was difficult for a WWII-style army to operate for any length of time when more than 100 km from a rail head or port. After the initial lunge, the Germans could not advance faster than the rate of gauge conversion. Maybe they couldn’t find enough railway engineers. Maybe Hitler thought his unconquerable will could trump mere logistics. Maybe the Wehrmacht high command underestimated the difficulty of working in the rasputitsa and the Russian winter. Whatever the reason, they couldn’t rebuild the railroads fast enough to take Moscow in 1941.
You can’t really understand the War in the East without understanding rail gauges – assuming a spherical Soviet Union is not the way to go.