The tenth amendment (states rights) has to do with powers not delegated to the U.S. by the Constitution nor prohibited by it to be reserved to the states or the people. Lincoln determined that the seceding states did not have the right to unilaterally withdraw from the Union, to which those states had bound themselves legally. He therefore took military action to proscribe their secessions. This is what the Ukrainian government has the right to do – make war on Crimea, which doesn't have an army of its own with which to respond.
There's no stable government in Ukraine, its president having been legislatively voted out of office accruing to what might be called the Ukraine Winter demonstrations and bloodshed. So, who has the power to order civil war against the people of Crimea? There may be an election in May but the problem is current. The Kremlin supports the annexation of Crimea to Russia so the affair seems to be beyond the power of any on-scene entity to neutralize it.
There are Ukrainian military operatives in Crimea, if only because its naval and coast-guard facilities/ships are stationed there. The Russian Black Sea naval facility is also stationed there in Sevastopol and the port is actually controlled by the Russians. It's said that there are 16,000 Russian troops stationed now in Ukraine whether they were already there or sent in from just across the border, where Russia has been conducting “training exercises.”
Russia has an active troop-strength of 845,000, while Ukraine's is 129,950. Russia also has armaments including nuclear weapons. It would be suicide for Ukraine to engage the Russians militarily, a useless loss of life, especially when the majority of Ukrainians in both the Crimea and southeastern Ukraine are Russians. Those who insist that such a war would precipitate World War III, dragging in countries such as the U.S., England and Germany are wrong.
The U.S., England and France have nuclear capability, so a “cold war” might occur, but not a hot one. If Obama, whose military acumen is zilch, should try to pull a “Libya” in the area, the Congress and the people (perhaps duplicating in the streets the Arab Spring of 2011) would put a stop to that if the military did not simply disobey orders. Obama, however, seems to have learned his lesson through being reminded by hordes of people of his total ineptitude vis-a-vis the “Syria Problem.”
Though the South, especially, and the North in the 1860s tried to gain support from England/France, both nations stayed out of it. The loss of much of the cotton production in the South, on which foreign interests depended for their textile mills, was the main factor on which either nation might have come to the South's aid. Ukraine, virtually bankrupt, has little to offer any nation in return for support.
NATO has indicated no military action from that quarter. The UN has no role to play except as a place for rhetorical axes to be sharpened through long-winded speeches made by self-important pooh-bahs. The most important EU nations get their gas from Russia via Ukrainian pipe-lines, so the chance that Putin could (and would) cut them off keeps them neutral. The Ukrainians could blow up the pipe-lines but then they would be out in the cold, not to mention forfeiture of potential funds from Europe to keep the country financially viable.
Obama speaks of sanctions as if he thinks Putin could not retaliate in many ways, such as simply nationalizing assets owned by Americans or closing the “northern route” the U.S. military uses to supply troops in Afghanistan, a huge segment of which originates in Latvia and goes through Russia. Obama and Kerry began their juvenile denunciations of Putin in obvious ignorance of all the ramifications involved. Now, all that's left for them is to eat crow and shut up.
And so it goes.