There are many factors. Demographically, there are parts of Ukraine that have an ethnic Russian majority, and where Russian and not Ukrainian, is the de-facto

There are many factors. Demographically, there are parts of Ukraine that have an ethnic Russian majority, and where Russian and not Ukrainian, is the de-facto language spoken. Among such regions you have Crimea, Luhansk and Donetsk. In the last census before the conflict, the 2001 Ukranian census, Crimea had 60% ethnic Russians, and Donetsk and Luhansk were close to 50%. In those regions there have been conflicts between ethnic Russians and ethnic Ukrainians in recent times. Some ethnic Russians in ethnic Russian majority regions would like to be governed by Russia. Another factor is that Crimea used to be Russian, and it was transferred by Nikita Khrushchev from the Russian SFSR to the Ukrainian SSR, and there are disputes over the region. https://en.wikipedia.org/wiki/Transfer_of_Crimea_in_the_Soviet_Union https://en.wikipedia.org/wiki/Transfer_of_Crimea_in_the_Sovi... Another factor is Russia trying to have a power projection to the Black sea through the Crimean peninsula. Russia annexed Crimea in 2014 but then Ukraine cut the water supply to the area (North Crimean canal/Північно-Кримський канал). Another factor is Ukraine surrendering their nuclear weapons and being left vulnerable. Another factor is NATO advancing East and Ukraine getting aligned and closer to the European Union and NATO. Another factor is Russia being a permanent member of the UN Security Council and thus having veto power over resolutions against it.