>>512009477 (OP)
The Soviet Union was weak until it got help from the democratic world.
Pre-ww2 Soviets lost a war to Poland and struggled to win a minor border conflict with Finland.
But when Barbarossa happened it became within the West's best interest to supply the Soviets with machine tools and technology and resources, and they used those things to build a colossal military industrial base, which they used to take the wealthy industrial heartlands of Germany and Czechoslovakia and pilfer German scientists to run their r&d for them.
The USSR was a generational power fueled by its war dividends from WW2, and collapsed as soon as those dividends were exhausted. Asking about the success of a country that failed to meet it's first centennial is silly.