NATO

 The North Atlantic Treaty Organization

This is the primary reason why Russia is going to attack Ukraine. The Soviet Union lost a majority of its territory in the 1990's after the break-up of the communist countries that were a part of the Kremlin.

Vladimir Putin knows Ukraine could possible join NATO which means - Ukraine would have the military backing of the world super-powers, which includes the United States.

If he attacks now none of NATO forces would be obligated to assist. But if Ukraine becomes a NATO member then an attack on them would be an attack on 30 countries.