Most significantly, Ukraine moved into an alignment with the United States and away from Russia. The Orange Revolution in Ukraine, from December 2004 to January 2005, was the moment when the post-Cold War mostly ended Russia.
The Russians saw the events in Ukraine as an attempt by the United States to draw Ukraine into NATO and thereby set the stage for Russian disintegration. The west had succeeded in dominating Ukraine, Russia would have become indefenible.
After what Russia regarded as an American attempt to further demage it, Moscow reverted to a strategy of reaserting its sphere of influence int he areas of the former Soviet Union. The great retreat of Russian power eneded in Ukraine.
In the 1962 Cuban Missile Crisis and now the process of that Crisis compares with what we face today over the Ukraine. Thomas Schelling of MIT in which he set out a scenario of a sequence of events and ironically placed near the Ukraine and to show that the USSR would accept and American nuclear attract without responding.
Just short of Russia itself in the Ukraine and as the Russians known some influential America have suggested that they should push forward "the gates of Moscow." They advocate what the British once called a "forward policy."