Three weeks ago, as of writing, there was a massive Russian invasion of Ukraine. This is a major event in the ongoing Russo-Ukrainian War that began in February 2014 when Russia annexed of the Crimea oblasts, and the eastern Donbas and Luhansk oblasts were taken over by pro-Russian forces. That annexation itself comes on the back of the Euromaidan protests following the decision of then-president Yanukovych's decision not to sign a political and free trade agreement with the European Union, instead choosing closer ties to Russia and the Eurasian Economic Union, contrary to the wishes of the Ukrainian parliament. Those protests led to the President Yanukovych being overthrown, but since then the ongoing conflict has led to at least 13,000 deaths prior to the recent invasion.
Indeed, since the break-up of the Soviet Union, Ukraine's political map has consistently shown a divide between the eastern and western sides of the country. There can be no doubt, for example, that there is a genuine desire among many if not most inhabitants of the Crimea, Donbas, and Luhansk oblasts that they would prefer to be part of Russia. This was certainly evident in Crimea where opinion polls consistently showed an overwhelming majority supported becoming part of Russia. However, neither NATO, the CSTO, nor Ukraine were particularly keen on a properly-conducted referendum on such matters (e.g., UN-conducted, international observers and peacekeepers, etc), and as a result differences of opinion were determined in a battlefield by force of arms instead. All of this was discussed in some detail in 2014 by your author; "The Ukrainian Crisis: Electoral History, Great Powers, and Self-Determination".