I think the general shift in mindset is this:
After WWII, the United States reoriented its entire strategic instruments of national power (Diplomacy, Information, Military, Economics) to counter the Soviet Union who was hell bent on world domination and the overthrow of the previous democratic capitalist world order. We drastically expanded our military footprint and alliance network to directly counter this, and ended up in conflicts throughout the world. These conflicts threatened our national stability (ie: Vietnam), but the leaders of our nation viewed that risk of domestic instability as necessary to fight the Communist threat.
After the fall of the USSR, we found ourselves alone as the unquestioned leader of global affairs. European countries continued to shrink their militaries in response to the dissolution of the Soviet empire and generally deferred to US global leadership on maintaining the status quo. In order to achieve this maintenance of global status quo, we have had to expend tremendous resources and mingle in regional conflicts that time and time again have strategically worked against our interests (Libya, Iraq, Afghanistan).
We are now faced with a ballooning debt (that neither party is really interested in solving), domestic problems we can't afford to properly address (healthcare crisis, drug epidemic, massive immigration crisis), and allies who are (generally speaking) entirely too dependent on the US for their own protection and have no interest in seriously addressing the problem we face as the global hegemon. The American people are mostly disillusioned with the idea of maintaining this increasingly expensive and increasingly ineffective status as global hegemon and would prefer to focus on our regional hegemony and domestic issues.
The concept that the United States (or any country for that matter) should sacrifice their own blood and treasure for a regional conflict on the other side of the planet with ancillary implications on what is best for the American people is a less than a century old concept. We could either continue playing the global hegemony game past our prime and flame out in embarrassment or go out on our own terms without seeing a collapse of the Empire.
Is Russia invading Ukraine bad? Yeah it is. Should we condemn it? Yeah, and I disagree with the Trump administrations inability to state that as fact. Should we continue to sacrifice billions of dollars of our treasure to prolong a war that Ukraine cannot win? No, I don't think so.
