By treaty the Russians are allowed up to:
on Crimean territory.
Establishing 16,000 troops into the Crimea wasn't an invasion. Just like when the US moves guys into Guantanamo, they're not invading Cuba.
So let's be careful not to heat up the rhetoric unnecessarily.
The Ukrainians have excellent reasons to hate and fear Russia, and the world would be a better place if they could finally become a free people - not ruled by Russia, nor by EU-sponsored oligarchs with their own militias, nor by National Socialists.
One way to make that happen would be to arm them and encourage them to enshrine something like the 2nd Amendment.
Another way would be for the US to become a large-scale NG exporter. When that happens a lot of the pathologies of Russia(=GAZPROM) will be contained.
Washington could make this stuff happen and there'd be no danger of war. Unfortunately - as we all know - they're myopic and corrupt.
Someone should go into the men’s locker room of the golf course clubhouse and explain the above to Mr. Obama. I’ll bet you a dollar it’ll be the first he’s heard of it.
I should add that the Russia-Ukraine Military Base Agreement (and its various extensions) isn’t the whole story.
Russian troops are clearly in control of the Crimea - they have established roadblocks, entrenchments etc. This level of military deployment is obviously not covered by the base agreement. Also we have the stories of Ukrainian troops being besieged in there barracks.
Russia can say that it is acting with the authority of Yanukovych - who is both a corrupt apparatchik of Russia AND the legally elected President of Ukraine.
It can also claim to be acting in accordance with the internationally-recognized right to self-determination - in this case by the people of Crimea.
How to unpick such a mare’s nest of legality?
The first thing to do is to clarify obvious facts - like the fact that the Russians have longstanding rights in the Crimea, established by treaty and by long historical provenance. It’s also clearly over-determined that the people of Crimea will formally secede to Russia.
This is de facto. It may also be de jure, or it may not. The fact is that Crimea is going to become part of Russia and its not morally obvious that it shouldn’t be.
The rest of Ukraine is another matter. East Ukraine is heating up: there could be/already is strife between EU puppets and Russian puppets, all with their own militias.
The right answer is to strengthen Ukraine - not as a EU or US client-state, but as a constitutional republic with an armed citizenry and able to buy NG for US prices rather than GAZPROM ones.