Crimea
Ethnic Background
https://www.businessinsider.com/this-map-explains-why-russia-is-invading-crimea-2014-3
In 1954, after years as part of the Russian Soviet Federative Socialist Republic (the largest state of the USSR), Soviet leader Nikita Khrushchev transferred Crimea to Ukraine, another Soviet state at the time.
After the USSR fell, Ukraine became a sovereign nation. It was decided then that Crimea would become an “autonomous republic” inside Ukraine, essentially giving it more autonomy over local issues while still being subject to the Ukrainian government.
2014 Timeline
Crimea was originally a part of Russia. Ukraine has a history of changing borders like Poland, Khrushchev gave Crimea to Ukraine. Ethnic Russians are the majority in Crimea.
https://www.theguardian.com/world/2014/apr/11/ukraine-russia-crimea-sanctions-us-eu-guide-explainer
Ukrainian President Yankuvich made overtures to Russia for better trade deals than with EU and become unpopular with Ukraine independents. After civil unrest, Yankuvich was removed from office.
With strong ties to Russia’s history and culture, the Crimean peninsula looms large in Russia’s national imagination, and has a large ethnic Russian population and significant pro-Russian sentiment. When the referendum rolled around on 16 March – denounced by Ukraine, the US and EU as illegal – officials reported a 97% vote to join Russia.
Russia leased ports for it’s Navy on Crimea. In some sense, sending in troops to protect equipment was to be expected.
Even BBC Concedes there was support for Crimea joining Russia.
https://www.bbc.com/news/world-europe-26606097