Russia would have been very easy to handle. No fly zone over Ukraine from the very start. The US could have closed the skies over Ukraine early on (as requested by Ukraine) and almost instantly put an end to the war before it got rolling. Putin would have thrown a giant tantrum, and then promptly backed down and sought peace talks. The US could have shot down every single jet & bomber Russia sent over the line.
War with Russia does not mean nuclear war with Russia. As we can clearly see from what Ukraine is doing to Moscow et al.
Also, an aggressive stance in fighting Russia, would have caught them early on when their military was entirely a disaster. The beating would have been an epic humiliation. Instead Russia has spent years re-learning how to wage war, which is not what you actually want.
Playing chicken is always risky.
Declaring a no fly zone would have achieved nothing. Sending air forces to enforce it would have done something. But what happens if American fighter jets engage Russians over Ukraine, while Russian air defenses support their air forces from their side of the border? Would Americans let Russians launch SAMs with impunity, or would they invade Russia, showing the world that Putin's justification for the war was in fact correct?
Maybe Putin would have chickened out when American jets show up, but you can't know that for sure. Which means you should probably decide in advance you far you are willing to escalate before you chicken out yourself.
"no fly zone" is one of those things that sounds obvious but if you actually think about it then it doesn't mean anything.
What mechanism gives the US the power to tell Russian planes where they can and cannot fly? The US cannot simply mark an area on a map. It would have to, at a minimum, deploy a series of surface-to-air missile sites to actually shoot down those planes that violated the map.