Ukraine and Georgia are seen as strategic buffer states against the EU-NATO by the Russian Federation in a way that the other states from ex-Soviet Union are not. Because of this, Gorbachev insisted that NATO not extend eastward to include these countries.
Now, according to Adam Tooze, a historian, the independence of Ukraine after the dissolution of the Soviet Union precipitated an economic disaster. GDP per capita halved between 1990 and 1996, recovering to 80% of its 1990 level by 2007, and has stagnated ever since. This contrasts painfully with its neighbours, Turkey, Poland and the Russian Federation.
Thus, given its struggling economy, the basic options before Ukraine was alignment with the Russian Federation, or with EU-NATO or balancing between the two. It was the latter option that was preferred until the colour revolutions in Georgia and Ukraine in 2007 when the decision began to look more stark.
In 2008, George Bush's administration attempted to force the issue. It encouraged both Georgia and Ukraine towards NATO membership and wrangled the other NATO members in the 2008 NATO Bucharest meeting to promising them membership. This realised the fears of the Russian Federation and when Georgia attempted to make good on this promise by first attempting to clamp down on the breakaway state of South Ossetia this precipitated a massive counter-attack by Russia. This was Moscow signalling to Washington and to regional actors not to make good on NATO's ill-judged promises at Bucharest.
This is more or less what is happening in the current crisis in Ukraine especially after one has noted the semi-autonomous Donbas province which is more aligned to Russia than the rest of Ukraine.