In many ways, this is kind of a Russian/Slavic Civil War. Ukrainians are a version of Russians, in fact, they can arguably be considered the original Russians and Kiev as the heartland of Russia.
Kievan Rus was the first real "Russian" civilization in Medieval Europe. The reason Moscow became the center was the Mongols. The Mongols devastated Kievan Rus and the Southern Russian states which had the more favorable land while they seemed to be content to tax the northern regions of Russia like Novgorod, Perm, Muscovy, Tver, etc.
Poland-Lithuania also ended up taking large swathes of what we know as Russia ruling Kiev, Smolensk, Minsk, and Ryazan at their height. They even conquered and Ruled Moscow for a time.
It was the rise of Muscovy that setup modern Russia and the Russian state is a descendant of this version of Russia. The Czars took back Novgorod, Ukraine, Smolensk, Ryazan, Kazan, and all the lands from Tartars, Poland-Lithuania, Sweden, etc. However, Ukrainians, especially in the Soviet era, have always been treated like second-class citizens by Russians. Also, a lot of Western Ukraine wasn't even a part of Russia until after 1945. It was original a part of Austria-Hungary and then Poland from 1921-1945. So cities like Lviv can hardly be considered Russian anymore due to outside influence.