Can I get a quick rundown on Crimea?

[Taem]

Thanks for helping me catch up to speed. One quick question, if Ukraine is not part of Nato yet, but only a candidate, then why is Obama so insistent upon Putin's noninterference in the matter? From my point of view, Ukraine is so close to the current Russia - not to mention it was once part of the USSR - that I'd expect Russia to react, same as we would if Mexico started a civil war in our backyard. This is the only thing I'm not understanding about this war so far, is our response to this issue. And if the USA is reacting to the threat of Russia taking over more land, where were we when China accumulated more land a few years back, and this more recent article I found when trying (unsuccessfully) to find info on China's land grab: Link