I've been following, though not intensely, the developments re Russia/Crimea; this report has what seems to be the latest. For reasons of time (and lack of expertise on the region), I'll limit myself to one point. The news reports have been referring to the Budapest Memorandum of 1994, under which Ukraine gave up its Soviet-era nukes and Russia pledged to respect Ukraine's territorial integrity. That's fine and all, but Russia is already required to do that under Art. 2(4) of the UN Charter. So the Budapest Memorandum seems to me, though I'm not an international lawyer, redundant on that point.
Also, one commenter at the WaPo said something like "why is the U.S. concerned about international law, given drones, Guantanamo, etc etc.?" That the G.W. Bush admin violated international law, and that the Obama admin may also be doing so in certain respects, is no reason to refrain from expressing concern/alarm when a country of some consequence appears to be violating one of the most basic norms of international society and one of the most basic provisions of the UN Charter. I'm aware there are all sorts of complications here, but the essential points, in terms of international law, seem reasonably clear. That's not to say anything about the policy questions, which I'm not going to do, not in this post at any rate.