I don't know if you're ascribing opposition to US intervention in Ukraine as leftism...or just wrong-headedness.
I understand that many people who do not undertake any serious study of the facts could do either, or worse: Accuse those who do not support intervention as supporters of Putin, Slavic Fascist Imperialism and Treason against the USA.
If anyone who is an advocate for either side in Ukraine thinks they have some sort of moral high ground they are either delusional or corrupt. Both sides are lying, ruthless murderers.
And Putin probably started it, at least the overt stages.
Many of the pro-Russia, pro-Putin posters here, are spouting the same anti-Americanism that we used to see in the free papers passed out just outside the gates of our military bases.
Russian sources are pasted all over FR, and I can go here https://www.wsws.org/en/ to read the pro-Russian arguments that I will see many of the posters here making with great passion and self righteous.
If you are old enough to remember the early 1970s, and were reading the left’s radical publishings, then you must see the similarity, actually, the duplication.