"Mommy, the boys next door are fighting again. Why can't they learn to play nice?"
"They're Europeans, Timmy. They don't know how to play nice."
European 'tribes' are no different from tribes in any other part of the world except that European tribesmen wear ties. This time it's Ukraine and the world, as usual, has turned to the United States to assume its customary role as policeman and haberdasher,
"You have to. You signed the Budapest Memorandum in 1994. It says you have to protect Ukrainian sovereignty."
No! Wrong! Not correct! Nyet, nyet!
The Budapest Memorandum, which was signed by the U.S., U.K., Ukraine, and Russia, says that, in return for Ukraine's giving up its nuclear weapons, the signatories agree to 'respect' the integrity of Ukraine. It doesn't say anything about 'protecting' that integrity. And, besides, it was never submitted to the United States Senate for ratification, i.e. not legally binding.
Russia may have breached the agreement but, unfortunately for Ukraine, there is no enforcement mechanism. (Before buying a used car or giving up your nuclear weapons, it's probably a good idea to read what you're about to sign.)
And under similar circumstances, by the way, would we behave any differently from the way the Russians behaved?
Suppose the United States were leasing a naval base from Mexico to protect the security of the Panama Canal (same way the Russian Black Sea fleet based in Crimea is leasing its base from Ukraine). And suppose the Mexican gov't were overthrown by one less likely to allow us to continue to use that base (ditto Ukraine). Does anyone doubt that the Marines would be crossing the border before you could say, 'Remember the Alamo?'
As they say in the Russian Tea Room, 'One man's taco is another man's pirogi.'