One of the top comments from people who don't pay much attention to world news is "why is this any of the United States business?". Glad you asked. In 1994 the United States signed something called the Budapest Memorandum. It is a pact between the US, UK, and Russia that says in exchange for the Ukraine getting rid of their nuclear weapons, all 3 countries agree to not only not invade it ever, but to defend Ukraine if any country ever does. 

Clearly Russia violated that treaty by sending 16,000 troops into the Ukraine against the government's wishes. Putin has tried to dance around that by saying the majority of the people living in the Crimea peninsula are Russian citizens who have asked for Russia's protection during a time which may be heading towards a civil war. But that is irrelevant. International law prohibits any military presence inside a sovereign nation to protect your citizens. Imagine if Mexican citizens summoned their President to send the military into Texas over some uprising. 

Russia wouldn't have made the move on the Ukraine for the same reason Mexico wouldn't roll into Texas. The host nation could defend itself. But knowing that the defense of the Ukraine rests upon the UK and US, Putin gambled that if he broke our treaty, we would be too weak to enforce it.

And if you think our President was unaware of that treaty, you underestimate his knowledge. Here is Senator Obama in 2006 thanking Ukrainian President Viktor Yushchenko for "making the Ukraine safer by allowing the US to safely destroy 400,000 guns and 1000 missiles". He then came back to Congress and lobbied for 48 million dollars in order to do that.

 

See how much safer they are now that they are unarmed? I guess Putin didn't think they were safer though so he brought over some stuff to keep them safe.