... though Trump is an amazingly bad player.
While walls were still crumbling all over Eastern Europe in 1991, Crimea held a referendum on autonomy, and voted decisively for the restoration of the Crimean ASSR as a subject of the USSR. This was not seen as a vote to stay in the Soviet sphere, but as the only possibly step toward autonomy. Then happily the USSR imploded.
Inside the Ukrainian independence, Crimea was now part of Ukraine, but retained its new autonomous status and pushed forward for more. In 1992 the Crimean parliament established a Crimean constitution, and in 1994 elected its own local President. At a new Crimean referendum in 1994, 78% voted for greater autonomy.
In March 1995 the Ukrainian parliament unilaterally scrapped the Crimean constitution and abolished the post of President of Crimea. Then the Crimean parliament was forced to define a new constitution, dictated by the Ukrainian parliament. So, there were not much love between Crimea and Ukraine, and Ukraine did very little to earn it. The Crimean wish for more independence were squashed, and the peninsula has not forgotten that...
Russia, Ukraine and the Western powers did nothing to dream up a possible broad solution. Since the Soviet collapse all the simmering conflicts of the old East block borders were just pushed under the carpet. And the nice vision of a common security structure, for all of Europe, is now squashed in favor of militarization all over...
Just a short lesson on forgotten history. A possible better solution for Crimea could have been to gain a neutral status, like the Åland Islands, an autonomous region of Finland. Former part of the Russian Empire, now a demilitarized and neutralized zone.