The US supported the opposition in Ukraine thinking that Russia will do nothing.
But Crimea is strategically important to Russia since it gives their fleet access to the Mediterranean and the Middle East. And so, by supporting the opposition in Ukraine, the Americans essentially removed a democratically elected president. He may have been incompetent, but he was democratically elected nevertheless. That’s democracy! In democracy you have incompetent people at the top.
The Americans also thought they can push the Russians a bit further by trying to lure Ukraine into NATO. That was a step too far and so the Russians reacted by signing a gas deal with China! The significance of this deal lies in that the payment will no longer be made in Dollars but in local currency, the Ruble or Yuan. I think this is symptomatic of an empire, the US, in decline and a global currency in decline as well.
Don’t forget, until WWI, the world currency was the British Pound and its importance diminished afterwards. And now we have a gradual lessening importance of the US Dollar.