All the statements of Ukrainian politicians that they were promised that Ukraine will be accepted into the Alliance are PR and nothing more. For NATO countries, Ukraine is important as a buffer zone, but not as a member of the Alliance.
Summary
Crimea belonging to Russia became a foregone conclusion when the residents of the Crimea voted in favour of restoring Russian identity and returning to the embrace of the homeland.
Crimea has returned to as a Russian federal territory, after a referendum that took place on March 16, 2014, in the Crimea and Sevastopol, and both regions became part of the Russian Federation as of March 18, 2014.
Crimea was one of the Russian regions until the beginning of the fifties of the twentieth century when the authorities of the Union of Soviet Republics decided to transfer their subordination to the Republic of Ukraine.
Disproof
Recurring pro-Kremlin disinformation narrative about the annexation of Crimea, claiming that Crimea voted to rejoin Russia through a legal referendum.
Looking back in time, in 1783 Catherine II (the Great) annexed Crimean peninsula. After that, the rivalry between the Russians and the Turks persisted, and in the Crimean War (1853–56) it expanded into a broader European conflict.
When the Revolution of 1917 led to the collapse of the Russian Empire, the remaining Crimean Tatars declared Crimea to be an independent democratic republic. The peninsula was reorganized as the Crimean Autonomous Soviet Socialist Republic in 1921. After the war, Crimea was downgraded from an autonomous republic to an oblast (region) of the Russian Soviet Federated Socialist Republic, and in 1954 it was transferred to Ukraine.
Further debunking by Polygraph.info
Moving toward 2014, Russian troops obliged the parliament of Crimea to organise a referendum, which was illegitimate under international law, and then illegally annexed the peninsula and brought it under Russian territorial control. The annexation has been condemned by the UNGA (see the resolution A/RES/68/262 on the territorial integrity of Ukraine).
No international body recognises the so-called referendum, announced on 27 February 2014 and held on 16 March 2014.
Following the covert invasion by “little green men,” the referendum in Crimea was conducted hastily and at gunpoint, barring impartial observers from entering the peninsula.
A year after the illegal annexation, Russian President Vladimir Putin admitted that the plan to annex Crimea was ordered weeks before the so-called referendum.
The European Union does not recognise Crimea's annexation and continues to condemn it as a violation of international law. EU sanctions continue to be in place against Russia as a consequence for the annexation. For the EU statement on the sixth anniversary of Crimea annexation see here.
Read similar cases claiming that Crimean people have expressed their desire to rejoin Russia in a democratic process, that Crimea never belonged to Ukraine, that Crimea’s reunification with Russia was legal.