That is the difference between the two. Putin is concerned over Russian security and Obama is not concerned for American security. I'm not saying Putin is right, there is no way Ukraine is a threat to Russian security even if it is totally in the EU.
It is tied to the Russian sphere by economics and tradition. Its politics are now leaning more toward the EU and away from Russia. Makes Putin very uneasy. Ukraine is the door into Russia and has been used as such in three wars that cost the Russians dearly (Napoleonic, WW I and WWII). Putin is not likely to forget that.
Obama doesn’t see this as a security threat to the US. I do (I know – big deal mminnick does) but others see it as a threat also. Charles Krauthammer Ralph Peters (LtCol Ret. US Army) see the adventurism in Putin’s move. If he can get Ukraine back as a Russian “Possession” then Georgia, Kazakhstan, Uzbekistan and the other republics established after the breakup of the Soviet Union will fall into line and the USSR is back in business just as the US is dropping out of business. Russia and China back in the driver’s seat in the world. That is what Putin wants and is out to get by hook or crook. If he could figure out how to dump China from the world stage he would not hesitate.