The political situation in Ukraine is politically and morally more complex than listening to the news media and politicians would suggest.
Politically, Russia wants to protect important national interests. The United States invaded both Grenada and Panama with less at stake (alleged concern for the well-being of a small number of U.S. citizens, mostly medical students, justified the invasion of Grenada to keep a pro-Marxist popular regime from power; trying to reduce drug imports justified the invasion of Panama) than Russia has at stake in the future of Crimea. Russia's concern include the well-being of Russian citizens and Ukrainian citizens who are ethnic Russians living in Crimea, the security of its major Black Sea Naval port, and the uninterrupted flow, through pipelines that cross Ukraine, of natural gas and oil exports. The United States arguing that Russia should not act assertively to protect its perceived national interests appears hypocritical, revealing that the U.S. perceives its national interests lie in keeping Russia weak and surrounded by unfriendly powers.
For similar reasons, the U.S., NATO, and the European Union badly miscalculated when they sought to incorporate former Eastern European nations into western alliances after the fall of the iron curtain. Those actions are akin to Russia or China seeking to incorporate Mexico and Canada into alliances as a way of limiting U.S. influence. The U.S. would, to say the least, feel threatened and respond aggressively, perhaps more aggressively than Russian has in the Crimea.
Morally, people have a right to self-determination. How granular is that right, i.e., at what level of disaggregation should people be able to form an independent state? This is not an abstruse philosophical question. Some communities in the United States would like to withdraw from the union because they feel disenfranchised by the federal and/or state governments; other communities in the United States want to secede because they wish to form homogenous ethnic or religious enclaves.
On the other hand, disaggregation can make the world a more dangerous place, result in the creation of a state that lacks the resources and population base to provide essential public services for its people (let alone defend its people and territory against aggression), and result in more rather than less prejudice (think of the racial cleansing that apartheid South Africa attempted).
Putin is not Hitler. Russia does not have plans for global conquest – those died when Communism did. The West lacks the wherewithal to stop Russia from annexing the Crimea without great loss of life – perhaps more loss of life than the Crimean population. Whether one views the problem through the lens of history, political realism, or ethics, the view is muddied.
The United States may object to Russia annexing the Crimea, may protest that action, may take diplomatic action, and perhaps even impose some sanctions, but the U.S. and its allies will do well to remember that in the final analysis stopping Russia would carry far too high a price tag to justify a war (for more on Just War Theory, cf. my book Forging Swords into Plows). Bellicose language is therefore inappropriate and unhelpful.