carl,

I don't think Ukraine can organize opposition with sufficient speed. Russian forces are already mobilized and the Russian Army isn't a divided organization like the Ukrainian one. I'm also confident Russia has prepared war plans for this kind of scenario -- does Washington and NATO? Given the geography, strategically speaking Ukrainian forces would be best positioned along the Dnieper and anchored in Kiev to prevent a river crossing - but the Russians wouldn't have to go that far for Ukraine disintegrate. Every Ukrainian position east of the river would eventually face encirclement and probably disruptions from Russian sympathizers. Ukrainian partisan activity would have to be urban in nature and Georgia and Chechnya should instruct everyone what that implies for the safety of Ukrainian cities during Russian combat operations.

There's also the question of defections in Ukrainian forces as well as uprisings in Russian enclaves, particularly Odessa. Russian airborne and naval landings would severely disrupt the support zones of Ukraine. Russia has also a large special operations and light infantry component dedicated to this kind of thing; disrupting enemy forces at all levels (there's already infiltration in Ukrainian cities for political agitation) as well as for police actions securing lines of communication. IMO, it's not a question of quality - but quantity, speed, and geography. Ukraine does not have the resources to fight a mobile war against Russia in open ground.

How fast could NATO mobilize in response and to what extent? As Fuchs mentioned, Poland and Baltic states would be fixed defending their own borders - that means US, German, French, and British forces. By the time they're introduced into theater and make it to the front, eastern Ukraine would be occupied. The key would be getting to the Dnieper before the Russians do, and in this situation, I don't think that's feasible.

Realistically, I don't think it's in Russia's interest to stage a larger intervention beyond Crimea, much less conduct a general invasion of the country.