Both sides are making choices - the US thought it could take Ukraine for NATO and break Russia apart in the process but it failed when Russia reacted quickly in crimea, now its using Ukraine as a sacrifice in an attempt to weaken Russia and having failed militarily is converting the war to a cheaper assymetric form of warfare.
You can fight your way into a dominant position and negotiate on your terms.
If you lose you might negotiate from a weaker position and trade off more than you would like or capitulate entirely.
NATO is getting ready to move in. Fighting on Russia's doorstep was a big mistake of for the US hegemony dreamers. US empire is in decline.....
