The territory of the Ukraine was part of Russia for over 300 years before 1991.
The Ukrainian people sought to be part of Russia after being invaded by the Mongols which eventually resulted in the Treaty of Pereyaslav in 1654. Even before that the Russian and Ukranian people share a common heritage. The Russian Empire considered Ukrainians to be ethnically Russian.
Ukraine was also part of the Polish-Lithuanian Empire for centuries as well... so They could just as easily be claimed by Poland or Lithuania...
The point is, just like Kuwait didn’t belong to Iraq, Ukraine doesn’t belong to Russia.