The Soviet Empire was made up of 15 Soviet Socialist Republics: Armenia, Azerbaijan, Belorussia (now Belarus), Estonia, Georgia, Kazakhstan, Kirgiziya (now Kyrgyzstan), Latvia, Lithuania, Moldavia (now Moldova), Russia, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan.
Ukraine is the second largest country in Europe after Russia. Putin has always entertained a fantasy of restoring the Soviet Union, starting with Ukraine (he admitted that many years ago). No I am not going to go hunting for proof.
This is the real reason for the present attack on Ukraine. He has a dream of walking all over Ukraine, then forging ahead.