IMO, whether we respond militarily is a matter of effectiveness. Conventional boots on the ground in Eastern Ukraine would be unwise. Even Kiev does not quite seem to know who all is loyal or not. But there are many other things we can do to make this a very negative venture for Putin, and also to assist the Ukrainian’s.
As for the Security Assurance(s), in part I agree - by strict legal weasel definition, the U.S. is not mandated by the Budapest Memorandum on Security Assurances (and follow-ups) to intervene militarily. (IMO, Ukraine should have just done an Israel and flat out rejected such terms, until they got a better deal, or hung onto whatever quantity of nukes they could control. However, they were under very heavy pressure to agree.)
Then again, if Putin were to chop off a part of, say, Latvia, who or what, would actually (immediately) physically compel us to defend Latvia? If we don’t live up to the treaty terms of being a NATO member, who’s going to punish us?
This is why I say the implications are moral, and long term, moral weakness destroys the physical: Not responding effectively to assist Ukraine in defending it’s territorial integrity, when we have the authority from the Budapest Memorandum (among other treaties and agreements) to do so, means no other nation under potential threat can depend on us either. It all becomes a matter of perceived “value of the threatened” at the time, from our end. Others will have to assume they may be on their own. No treaty with the U.S. is worth more than the paper it’s written on, if the U.S. is seen as unreliable.
This is not just a matter of our government, it’s also an assessment of the American people by other governments.
I am not legally obligated to defend my neighbor’s wife, if I see someone attacking her. There is no “promise” or written agreement at all! And I might get myself killed. But in a world where I cannot simply move away, if I do not help, no one would ever trust me again, nor should they, and my life will go accordingly.
As for Ukraine, I think you may be discounting what is wanted by the now clear majority of the people, taken on a national basis. That is, with Crimea gone to Russia, the rest of the country is not going to be electing pro-Russian or even neutral Presidents or Parliament majorities ever again. Even if a neutral gov’t could be elected, given the probable economic environment, a Maydan 3 with real radicals coming to real power would loom in the future.
Besides I really do believe Nato/Us/Eu are going to protect the Nato country's via a good 'show' ..so that the Nato country's don't get their pants in too tight of a knot.... Imo, and we know how opens go.... it was already determined to let Putin handle the situation if it got out of hand, once they met in Geneva..... Though personally I think it was in the cards Putin would go in and save the day before this whole thing even began to come into view for the audiance..... His “cut” of the pie was Crimea.
Trouble is they misjudged the citizens of Ukraine..and the Russians within as well...not to mention the Nazi element and fringe groups. (Nation leaders always figure they can control the situation with the least carnage possible.)
Your see many of those leading the groups can't even control their own...so everybody has to keep picking up the pieces and blame someone else when someone is killed, kidnapped or thugged up. It's chaos there because in reality Ukraine is generally just that without enough thugs in their Government to keep the peace.
Further they don't understand negotiations let alone democracy and what that means or entails....Heck when Kiev Gov. sent their so called Anti-terrorist thugs to Donestk the first thing they asked the Gen. leading them was...who was he calling terrorists?..... and the first thing the people in Donestk asked was,..."who are you calling the terrorists"...he wouldn't/couldn't answer...so they went after him and roughed him up...Then trying to escape he finally was able to slip through an exit door that his team had made for him, ( he wouldn't go over the fence to escape because "he was a General"... and then of course “Like a good politician” ,knowing it was filmed, waved and smiled to the crowd as he escaped their clutches. (that whole event is on video)
That was just one incident of the mindset in Ukraine, and it ebbs and flows like a yo-yo on a string.
So you see Ukraine really cannot Govern their people without a stronger and wiser hand...they simply aren't ready for that. Their only options have been the same Oligarchs which keep robbing them of ever getting on their feet. The EU and the US see that now, and did early on when they attempted to guide the protesters and what happened?...many died and they burned down their city square and several buildings.
Putin's 'advantage' is he knows well the people....unlike the Eu and US. Russia's dealt with them for centuries so he has a pretty good idea how to handle them. ...he knows they are a highly emotional lot, and in fact addressed that in his Q&E directly.
I'm not sure the EU/US even cared to understand the nature of the people before stirring this nest....or if they did were willing to take the risk.
Putin's going to come out the hero in this....he'll be the one to calm the waters in the end...and the journalists will keep writing the stories about what a thug Putin is....but the truth is we looked like idiots, because that is what we have governing our country...and the EU isn't much better if that. How far Putin can go is pretty much as far as he wants... how far he will choose to go remains to be seen...but I doubt he'll ruin his revenue flows for the sake of taking on another welfare state, anymore than the EU/Nato was willing to add another one to their failing states already putting a drag on their "Kingdom".