The United States, without a shadow of a doubt, deployed its troops in Grenada, having said that there was a threat to American citizens there. In addition, not that long ago, America said that the use of the army against one's own people was illegal. The US wanted to invade Syria for that reason, where Assad "was using the army against his people."
As for Ukraine, the USA, of course, does not see any parallels, just as it was with Kosovo and the Crimea. According to the United States, the separation of Kosovo did not violate international law, although the rest of the country was against it."
Russian Foreign Minister Lavrov explained in one of his recent interviews, in which case Russian troops could be deployed in Ukraine. "The deployment of troops can be carried out in case of direct infringement of the interests of Russia and Russian citizens," the minister said. In a nutshell, "If we are attacked, naturally, we will defend ourselves." According to Lavrov, "aggression against Russian citizens would be considered an attack against Russia."
No matter how many foreign warships there are in the Black Sea, one single volley from them will mean retaliation that will instantly escalate to war between Russia, the United States or NATO. Such a war is likely to be global and nuclear. Is Obama ready to sacrifice American cities for Ukraine? Is NATO ready to burn millions of its citizens for the glory of Kiev? Of course not.
Russia has also made very clear that if ethnic Russians are attacked in Ukraine, Russia will step in militarily. The question isn't whether but just how much ethnic Russians in Ukraine will have to endure before direct Russian military intervention. I believe Russia is making it very clear that it won't stand for much more from Kiev (the United State's handpicked government there; its proxy).
Mr. Putin won't allow the many fascists in Western Ukraine to have the upper hand over Russian ethnics in Eastern Ukraine, and there's nothing the US can do to stop him regardless of what John McCain or others think to the contrary unless the US wants to risk Mutually Assured Destruction (MAD) via global, thermonuclear exchange that will quickly render the planet all but uninhabitable for human beings.