I'm not particularly concerned about Russia militarily threatening the US, but a strong and emboldened Russia putting pressure on Europe is not favorable for us. Eastern Europe generally speaking has less money and power, and therefore with have trouble defending against Russia. As those nations fall, Russia gains land and resources, which makes them more of a threat to Western Europe. Even at a near stalemate in Ukraine, resources are flowing from us and our allies, bottlenecking supply chain, production, logistics, etc. This leads to challenges in receiving resources, and the price is higher if we can even get shit. If you stop thinking right there, I guess I can see how people might oppose aiding Ukraine. But people who think things through realize that Western Europe in direct conflict with a stronger Russia will only make those exact issues even worse. And decent people don't even need to realize and understand that because the moral implications of indifference to the massive amount of dead civilians is plenty of reason to be on board. But even a sociopath should be able to understand the concept of "a stitch in time saves nine."