The Russian Empire acquired a sixth of the Earth by integrating conquered elites rather than their peoples, and nationalism is what made that model fall apart.

The elites would be integrated, and the empire was based on the state loyalty and the state integration. But once you bring in the factor of nation and nationalism and language, then, once in a sudden, the whole model of the integration of the elites, irrespective of their language, religion, and culture, starts falling apart.