Our military, of the United States that is, is not prepared to fight and defeat a near peer enemy military such as the Russians. When I entered the US Army in 1991 and reached my first duty station our forces were returning from Desert Storm. When the United States deployed its military forces during the lead up to Desert Shield/Storm its doctrine strategy was mass force on force or division vs. division combat. At the time we fully expected Saddam's full military might to be near peer and to put up a "worthy" challenge to our own military. In short, we went into Kuwait as if we were facing the Russians. We used artillery en masse, flew countless air sorties to soften up the enemy, and went about fighting that war as if it were a reenactment of WW2.
Up to around 1998 we (the US Army) continued to train to fight division vs. division sized engagements both in desert and (in a European style environment) against the Russians (and Chinese, perhaps on the Korean Peninsula). In other words, the US military of that time was ready, willing, equipped and able to fight near peer adversaries, such as the Russians.
All of that changed after 9/11 and the kickoff of the global war on terror. From about 2001/2002 US Military doctrine changed . . . for the worse. We very rapidly (in Theater, no less) trained our forces to fight insurgent enemy forces, including unconventional terrorist guerillas, who were often decades behind our own soldiers in gear, weapons and general capabilities. We taught our military forces and their tactics to become dependent on absolute friendly air superiority, and we gave them a false sense of confidence in the process. After drilling into our soldiers' heads for twenty years how to fight under friendly controlled skies and zero enemy artillery or armor capability, we've pretty much ruined them for the bigger, division on division fights against a nearly equal near peer enemy.
Army doctrine of the last twenty years has essentially been a combination of rapid advance under the cover of friendly fighter/bomber/air cavalry who were always there to use for airstrikes and fighting from easily/completely controlled forward operating bases and green zones, many of which could be very easily resupplied. In other words, in most cases, our conventional military learned to fight as a reactive, occupation force who ALWAYS had allied air power on speed dial and almost always had safe, static walls to retreat behind.
If we get into an actual shooting war with Russia in Eastern Europe the skies will not only be contested for a very long time, but for the first time since the ******* Vietnam War our military men and women will have to worry about being on the receiving end of airstrikes and mass artillery barrages. They will have to unlearn how to be a dominant force of occupation fighting against terrorist and insurgent rabble, and how to deal with an enemy who is pretty much as capable as they are, both on the ground and in the skies. We're just not ready for that, and we need about decade to un-**** ourselves and retrain our forces for near pear combat on a massive scale.
Our politicians and military leaders alike are well aware of everything I've mentioned above and will sure as hell take it into consideration before jumping into war against Russia. At least . . . I hope the Biden administration isn't that suicidal—suicidal enough to challenge a near peer military in an all-out war. On top of all that, there's that little issue of us and the Russians both being nuclear armed powers.
Let Russia have Ukraine—if it comes to that. But it won't come to that. Chances are very, very low Putin sends in the troops.