You miss understand imperialism. Imperialism is not simply whenever a war is not about national liberation. Imperialism according to Lenin is about the need to export capital. Russia exports mostly goods and resources. The world Bank lists Russia as developing nation. Foreign direct investment outflows out of Russia are negative. Russia is not imperialist. The capitalist class in Russia would make better profits without the war, even if they had lost the Ukrainian sphere of influence. The reason Russian capitalists need this war it to defend assets inside Russia from NATO imperialists. In repeated negotiations to prevent the war, NATO did not accept yes as an answer from Russia to their demands.

On the other hand, US capital would not survive without the forever wars. US absolutely needed this war, that's why they started it. Remember, Putin wanted Russia to join NATO and become subservient under US. US didn't accept, because of the absolute imperative to have war.

But granted, Russia was probably on the edge to become imperialist. It's not as straightforward as in Iran's case, where I think the case could be made much more easily to speak of a war of national liberation.