We also promised the Russians in that memorandum that we wouldn't be intervening in the Ukraine, and we most certainly did. The memorandum is essentially like a contract, and the general principle of contract law is that breach by one side allows the other side to stop performing.

Your characterization of the situation is also flawed: the Crimean referendum is not a "bogus referendum", nor are parts of the Ukraine being "stolen". These are parts of the nation that no longer want to be parts of the nation. If memory serves, we supported "bogus referenda" and the "theft" of Kosovo from Serbia, East Timor from Indonesia and South Sudan from Sudan. In Serbia, the US decided to bomb Serbia for a few months with no real authorization from any international bodies, and in the other cases the US used threats of the same. We also invaded the sovereign state of Iraq on the flimsiest of pretexts.

The question that I then pose is this: if the US can do as it wishes without any international authorization, why can Russia not act to defend its core national interests (naval base at Sevastopol) and its citizens (many in Crimea already have Russian passports), as well as people who are Russians but not citizens solely because they ended up on the wrong side of an arbitrary border in 1991? I have yet to hear a satisfactory answer to this question.

The simple fact is that if US citizens and the US national interest were as directly threatened as Russia's is right now, we wouldn't even be as careful as Russia has been - we would have already overthrown the country and invaded it.