After Russia moved to raise the price of the natural gas it supplies, Ukraine warned that it may revise the contract regulating rent paid by the Russian government for the Black Sea Fleet bases in the Crimea. The Russian defense minister, Sergei Ivanov, responded with a thinly disguised threat to take the Crimea away from Ukraine. A revision of the rent contract, he said, might lead to renunciation of the 1997 agreement by which Russia recognized the 1954 borders of Ukraine.
Several Russian legislators chimed in, but unlike the defense minister, they were quite explicit. "Sooner or later the Crimea will be reclaimed," one Russian Duma deputy said. "To make the Crimea again part of Russia will be an absolutely right decision," said another.
Like other areas of the "near abroad" there are ethnic Russians stuck on the other side of the border who could provide an excuse for Russia to intervene. Add in the Russian view that we engineered the Orange Revolution that brought in a pro-Western government and we could have new European crises in the next decade.
I imagine Ukraine would like to speed up talks to join NATO.