Crimea didn't happen out of the blue. Russia wants control of it's own border and it wants a buffer from NATO.
The US has declared the entire western hemisphere is it's buffer.
Putin has said the same thing for 18 years now. No NATO in Ukraine.
The US and the west have pushed NATO further and further east.
Obama literally toppled the government of Ukraine which directly resulted in Putin taking Crimea.
Most posters here will be shocked to discover Crimea was part of the Ukraine.
The US topples Ukraine's government and Putin immediately takes Crimea. He said he did it because the naval base at Sevastopol that the USSR built would never be a part of NATO.
Bad foreign policy caused this. More bad foreign policy won't help either.