Even the original change of government in Ukraine was done on our instance and hence we owe a certain responsibility to those people for the consequences. Whether we should have made that representation or not is a wee bit late to debate because the question now is what happens around the world if America is seen to renege on its promises when those promises are so vital and fundamental as they were to Ukraine?
You have a similar problem of reliance with respect to the Iran nuclear deal, other European nations were involved and they also acted in concert with the United States, one might say in reliance on the United States' commitment. At least in this context, we can say that the United States vital interests are clearly involved and if we renege it will be on a calculation of that interest. Anyway, there is an argument to be made that Iran has already broken the deal opening the way for us to declare it void.
I say all this as one who has long and often in this forum argued that the United States should approach its international relationships from the basis of its own self-interest rather than from altruistic impulse. Here however, the faithfulness of the United States is at issue because the mistakes have already been made and positions have changed in reliance.
Finally, I refer you to this VANITY which perhaps puts our relationship with Russia in a different and larger context, a context which may well reshape the thinking of all of us on how to treat with Russia over Ukraine. At any rate, it should put to rest all of these half-baked assumptions about the integrity of Vladimir Putin or the benign nature of Russia today.
First, Ukraine was fully aware of US Constitutional law when the agreed to the those provisions.
Second, to suggest the United States has any obligation to Ukraine as a result, or Iran, is a ludicrous contention in law.
You say the United States agreed with Russia and Ukraine. I say there was never an agreement beyond what Bill Clinton could personally guarantee.
And everyone was fully aware of such before they signed.
The United States loses NOTHING in credibility or stature as a result.
If Iran and Ukraine think Clinton and Obama feckless, all the better.
Lesson learned.