You're going to get real lonely real fast if you think that supporting Ukraine and being a leftist are mutually exclusive.

Ask yourself this: Does standing up for an imperialist, autocratic fascist really align with my progressive ideals?

Just because he hates America, doesn't make Putin your friend. And Putin's people have gotten very good at spreading their propaganda through far left spaces, because they know how desperate people are in these spaces to latch onto anything that smacks of American Imperialism.

And to be clear, America is an imperialist power. No one is questioning that. But you have to look at the facts of the situation, not just reflexively assign heroes and villains based on your pre-existing feelings.

Every country has a right to its sovereignty. The "NATO question" is a Russian lie (they never brought it up when they annexed Crimea, their recent actions have actually pushed more countries into joining NATO including ones on their border, and Ukraine actually did offer to make never joining NATO a condition of peace but the Russians refused because they had no real interest in that), but even if it wasn't it still wouldn't justify invasion. Ukraine has the right to negotiate whatever treaties they want, because that's their right as a sovereign nation. By the exact same justification Russia could have invaded Canada to prevent us from joining NATO.

If the US (or Canada) invaded a country to stop them signing a treaty, would you feel that was fair and justified, or would you consider it another example of Western imperialism?