There has always been a regime of sanctions against Russia and, realistically, there will always be one. The sanctions are now so ingrained in the legal systems of the US and other countries that they will be impossible to remove. Russia will not ask for the sanctions to be removed or repent for acts it did not commit.
Summary
Ukrainian authorities could have foreseen that Russia would not permit NATO’s fleet in Crimea. After the Euromaidan in 2014, a member of the Verkhovna Rada (parliament) called for a Ukrainian-Russian agreement on the stationing of Russia's Black Sea Fleet in the Crimea to be disavowed. He said the Russian fleet should be driven out of there and NATO ships should be let in instead. It was Ukraine's mistake and the prologue of the Donbas war.
Disproof
This is an oft-repeated half-truth from pro-Kremlin media, claiming that Moscow’s reluctance to have NATO troops in Crimea was the only cause of Russia’s illegal annexation of the Ukrainian peninsula.
In reality, this is not the only cause for the annexation. Moreover, it can be hardly seen as the main cause because Russia had been continually trying to gain control of Crimea since 1991 when the Soviet Union broke up and Ukraine became independent again. In 1995, the “Agreement between Ukraine and the Russian Federation on the Black Sea Fleet” was signed, according to which Russia took 81.7% of it and Ukraine 18.3%. Another agreement prolonged the stationing of Russia’s Black Sea Fleet in Crimea for another 25 years, so that Russia would maintain its military presence in the Ukrainian peninsula until 2042.
From 1921 to 1954, Crimea was part of the Russian Soviet Socialist Federative Republic. The peninsula became part of the Ukrainian Soviet Socialist Republic in 1954. At the moment of the breakup of the Soviet Union, it remained in Ukraine, and Russia had to put up with it. In spite of that, Moscow was fomenting separatist sentiment in the peninsula by financing pro-Russian organisations and political movements, hoping to gain control of the peninsula, which Moscow thought belonged to it by right. In 1992, Crimea’s pro-Russian leaders attempted to stage a referendum for independence from Ukraine, but they failed. In February 2014, armed people without identification marks, who later turned out to be regular Russian troops, entered the Ukrainian peninsula, seized the government and parliament buildings, after which an illegal referendum on independence from Ukraine was successfully staged on 16 March 2014.
No international body has recognised the referendum, the UN General Assembly adopted a resolution which stated that the referendum in Crimea was not valid and could not serve as a basis for any change in the status of the peninsula, and the European Union adopted a non-recognition policy towards temporarily occupied Crimea.
Russia had been trying to impose control over Crimea by military means or otherwise since 1991. Although Moscow justifies the illegal annexation by a possible deployment of NATO troops there, it was rather a pretext than a real cause. The ongoing war in Donbas is a consequence of Russia’s annexation of Crimea rather than of the victory of Ukraine’s Revolution of Dignity.