Ukraine and Israel are used in similar ways by the US Empire, to attack their geopolitical opponents. Russia seeking to annex the Donbass region, a region that had already seceded from Kiev in 2014 after the right-wing coup overthrew the president they supported, isn't imperialism in the sense that Marxists are talking about.

When Marxists speak of imperialism, we mean a specific stage of capitalism by which finance capital becomes dominant and international extraction becomes economically necessary for continued existence. Russia is fighting against that system internationally, because the US Empire is keeping them boxed in and wishes to re-imperialize Russia like it was in the 90s, before the nationalists overthrew the liberals.

Marxists do not have an issue with anything anyone can call imperialism, but with imperialism as it exists as a necessary and scientifically observed phenomena of late-stage capitalism, because this phenomena is the biggest obstacle to global socialism. Russia is helping Iran, Palestine, Cuba, Venezuela, and more in resisting the US Empire, because Russia benefits from the end of a US-dominated world. Western Europe is vassalized by the US Empire, which is why they side with the US Empire. They benefit from this system of plundering the global south, Russia does not.