c) Russia sent in armed men and Crimea declared its independence to become part of Russia.
In 2014, Crimea, a region in Ukraine, experienced a series of events that led to its annexation by Russia. The situation unfolded as follows:
1) In late February 2014, political unrest and protests erupted in Ukraine, leading to the ousting of Ukraine's President Viktor Yanukovych.
2) Following the political turmoil, unmarked armed men, believed to be Russian military personnel without official insignia, appeared in Crimea. They took control of key infrastructure, government buildings, and military installations in the region.
3) In early March 2014, a controversial referendum was held in Crimea, where the residents were given the option to vote on whether Crimea should join Russia or remain part of Ukraine with increased autonomy. The referendum took place under the presence of Russian military forces.
4) The majority of voters in Crimea chose to secede from Ukraine and join Russia. The results of the referendum were disputed by Ukraine and the international community, which viewed the process as illegal and lacking legitimacy.
5) Following the referendum, Crimea declared its independence from Ukraine, and shortly after, Russia officially annexed Crimea, incorporating it as part of the Russian Federation. This move was met with condemnation from Ukraine, as well as many countries internationally, who considered it a violation of Ukraine's sovereignty and international law.