The US, UK, etc. actually did not "promise" anything regarding Ukrainian borders.
It's helpful to read the actual memorandum:
http://en.wikisource.org/wiki/Ukraine._Memorandum_on_Security_Assurances
Note that (a) it's a memorandum, not a treaty - in other words, a fancy diplomatic term for a press release, and (b) nowhere does it say that the US, UK, etc. are legally obligated to defend Ukrainian borders. It merely says they "confirm" and "reaffirm" and such.
Of course, the UN charter, etc. are a different matter. But then again, we're on the subject of international law, which is ultimately meaningless. The only thing that matters in inter-sovereign relationships is force, be it military, economic, etc. That's the reality and will be for the foreseeable future. There is no global organization that is empowered to end conflicts and this is an excellent example. The UN is powerless as long as Russia owns a Security Council veto.
The line about "protecting our ethnic nationals" is one of the oldest in the book and it's always nonsense. As a point of reference, it was a favorite of Hitler's.