Wow, to read the posts here is quite disheartening.
Let's imagine an alternate world:
In 1918, the United States, Canada and Mexico unite to form the American Soviet Union (ASU from now) under the leadership of dictator Woodrow Wilson, to counter the French Empire that conquered Europe. Then, in 1954, ASU dictator Truman transferred New England from the US-Soviet Republic to the Canadian Soviet Republic, for reasons nobody understands. Imagine also, that Norfolk is in New England and is not only the biggest Atlantic military port, but the ONLY Atlantic military port. Now imagine that the ASU dissolves in 1990, with New England remaining part of the newly independent Canada, but with a long-term lease for the Norfolk military base.
And now, in 2014, the pro US Canadian president is overthrown by mass protests in Quebec, and the new Canadian government wants closer military, economic and political ties with the French Empire, over strong opposition from the people of New England, who see themselves as (US-)Americans first. The new interim parliament also passes a law that would outlaw teaching English at school and made French the sole language of Canada, but the interim president wisely vetoes this law. And imagine that the interim government grumbles about terminating the Norfolk lease, thus depriving the USA of it's only Atlantic port.
A United States that invades other countries over alleged weapons of mass destruction would invade/liberate New England about as fast as Putin invaded the Crimea. And this analogy is pretty close to the reality. The Ukraine did not control the Crimea since the 11th century, when the Mongols invaded Kievan Rus, razed Kiev and settled Sunni Tatars (a Mongol people) in the Crimea. Later the Crimea would be part of it's own Khanate, becoming part of the Ottoman Empire even later, until the Russian Empire conquered it in the early 19th century. The Crimea became part of the Ukraine Soviet Republic in 1954, when Khrushchev transferred from the Russian Soviet Republic for seemingly no goo reason.
It is not unlikely that the planned referendum over Crimean independence would have passed without Putin intervening. Of course now any such outcome is suspect, but what Putin is doing is actually pretty sensible in the view of Russian self defense.
And of course saying that PUTIN is the cold warrior here is quite something. During the fall of the Soviet Union there were several agreements between the West and Russia. For some concessions like allowing Germany to reunite and become part of NATO, the Alliance promised the Russian not to expand eastwards into Russian borders. Here we are 20 years later, with nearly all former Warsaw pact members being part of NATO, even the former Soviet republics of Estonia, Latvia and Lithuania are members of the Alliance. President Bush opened new many new military bases around Russia, and Dick Cheney had plans not only to wholly crush Russian influence outside Russian borders, but he even wanted to dismantle Russia itself.
Russia is rightfully feeling threatened, and that logical actions like securing it's much shrunken sphere of influence are seen as Russia restarting the cold war is quite something. Especially from a country which had the Monroe doctrine, which declared the whole Americas as the US sphere of influence. Russia by contrasts only seeks to retain it's influence over their border countries.
Now I am certainly not in favor of what Putin did, but the hypocrisy of some people is sickening. And Putin is not making this move because Obama somehow makes America look weak, he is doing it because he sees vital Russian security interests in danger, in the face of an ever expanding Western influence.
And to answer insipid posts about Europe not caring enough or Putin having the EU by it's throat: Over 40% of Russian trade is with the EU. If the EU and Russia stopped trading with each other, the Russian economy would collapse, and the EU states would suffer mightily, although the Greeks would be much less able to pay rising energy prices than say the Swedes. It is mutually assured destruction, Russia is as dependent on trade with the EU as the EU is dependent on Russian oil and gas.
And of course the neocons are screaming to do something, but I cannot see what should be done. The US can suspend trade with Russia, with Russia accounting for 2% of US trade. But the EU cannot do that, the same that the US cannot stop trade with China without crashing it's own economy. Making the Ukraine a battlefield will end worse for the Ukrainians than simply letting Russia take the Crimea. It will not set a precedent, since the only other non-Nato members in Russia's vicinity are Belarus (good riddance!) and Finland (look up the Winter War for why Russia is unlikely to try to invade Finland again), which means any further expansion would lead to a war nobody wants, including Putin.
Diplomacy is the best course of action, and it should be noted that Russia has not yet annexed the Crimean Peninsula.