This Foreign Affairs piece (pdf) is interesting and prescient thoughout, here is one excerpt:
Despite some testy moments, relations between Russia and Ukraine have generally been stable since the Soviet break-up. There are, however, good reasons to fear these relations might deteriorate. First, the situation between Ukraine and Russia is ripe for the outbreak of security competition between them. Great powers that share a long and unprotected common border, like that between Russia and Ukraine, often lapse into competition driven by security fears. Russia and Ukraine might overcome this competition and learn to live together in harmony, but it would be unusual if they do.
Most of all, Mearsheimer argues that Ukraine should have kept its nuclear deterrent. Here is my previous post on that topic.
For the pointer I thank Shivaji Sondhi.