Private ownership of industry, which Italy and Spain and Germany all had, yet the Soviets did not.
Anyway do you contend that the Soviet Union, which referred to WWII as "the great patriotic war" was not nationalistic? I say they were, every bit as much as Spain and Italy and for the same reason: they dictators in charge needed to create a mythology around their nations in order to justify that their state was legitimate and strong so they could hold onto power. Indeed, the first thing that happened when the soviets collapsed was the splintering of the Soviet Union into its component parts.
As I've said though, the thing that set the Soviet Union apart from the fascist states of Italy, Spain, and Germany was the private ownership of industry.
The biggest reason why Soviet empire collapsed is because there was no freedom to get rich through entrepreneurship. That stifled innovation and productivity. It was central planning all the way.
That is the same reason why Nehru’s India remained mired in poverty. Central planning and control of industry. Only when Nehru’s congress party lost to BJP, India began its march towards prosperity.
Simply put, the Soviets planed to conquer the world. In the Soviet Union, they incorporated numerous ethnicities.