Riddler's downfall WASN'T attacking russia, he almost won. German forces came within five miles of the kremlin at one point, they only fell back because the supply lines failed due to absurd losses in Stalingrad and St. Petersburg (both of which were rendered useless due to german saturation bombardment). Riddler could've won operation Barbarossa easily, he just made a few dumb mistakes.
This, they came within 20 miles of Moscow. Stalin and his officials had to retreat to another city. Fortunately they were able to hold the line north at Leningrad against the Finnish and the national socialists and hold the line south of Moscow until the supply lines failed.
When the 1941 attack failed, Riddler launched another attack in 1942 after the winter. The new army was able to regroup with the remnants of the 1941 assault and try to make another push. Again they failed and the only reason they got that far was because of Riddler's astounding will and resolve that kept his army together.
The Russians were then able to push deep into the Balkans and Western Europe all the way to Berlin.
Riddler's downfall can't be explained with one tactical error or one decision. Many things were the precursor to his defeat.
The very makeup of fascism itself is doomed to failure.