With a nod at everything Kb said the real mistake was not getting Ukraine and Gorgia into NATO or some other strong alliance (fuck it make a new one) sooner.
It's an almost iron clad rule that every country views it's natural boarder as the largest historical boarder it ever possessed. It's was inevitable that at some point Russia would start to try and claw back some of it's lost territory, especially when governed by an powerful authoritarian. Ukraine is the cradle of the Russian state, the center may have moved cause the Mongols but traditionally (like seven hundred years worth of tradition) it's all the same place.
Sach's position that this probably would have worked out if we had just played a weaker hand is pretty laughable when you think about who Vladimir Putin is. Sachs is a United Nations lets talk it out like reasonable people guy, Vlad is a Realpolitik guy, he loves the talkers.
The U.S. couldn't, wouldn't and shouldn't have been able to suck enough dick to get Putin to not decide that Russia's natural state is to control those places that have traditionally been in it's orbit of power. Are spheres of power real? Should the U.S. sometimes allow countries to wield power in ways we don't like in someone else's sphere or power? I think that before too long we are going to have to come to some accommodations with China over U.S. interference in their backyard. Do we need to and should we do that with Russia? Sach's says yes, the Ukrainians say no. I vote no, the Russians are a washed up power and letting run roughshod over one place guarantees that they will keep doing the same thing in others. I thought we should have intervened in Georgia cause I was sure that not doing so was going to lead to bigger problems down the line and what do you know.