The problem with the Left (and most on the Right) is that they prefer their own set of facts. They ask "Why didn't Trump stop them?" The better question is this, "Why didn't Obama stop them from taking Crimea?" I would argue that Russia was encouraged by Obama's inaction.
That aside I would also argue that this War would not have started had Zelensky NOT STARTED SHELLING THE EASTERN OBLASTS IN 2014 Russia would not have invaded. The Eastern Oblasts speak Russian and identify as Russian. Included with the shelling was a covert Sniper War. Russia invaded to put a stop to it. No, I don't support War, Russia, or Ukraine. But it's all about the money the Banks are getting rich and young men are dying. It's just that simple.