The lands that are now Russia and Ukraine were united as a part of Kievan Rus', until the state fell in the late 13th century.
Symbolically important time in the history of Russia and Ukraine. While Russia was its own nation, Ukraine was under the control of multiple other countries. The people of Ukraine rebelled, and created their own nation. Shortly after, the newly formed Cossack Hetmanate (Ukrainian Government) declared loyalty to the Russian Tsardom through the Pereislav Agreement.
Through the Truce of Andrusovo, Ukraine officially became a part of Russia. This was an important step towards the re-unification of the members of Kievan Rus' and of the Ukrainian, Belarusian and Russian peoples.
The Cossack Hetmanate became less necessary over time, as Ukraine was now a part of Russia, and it was abolished by Catherine the Great in 1764.
Due to the collapse of the Russian Empire, Ukraine was temporarily separated from Russia. However, the new Soviet Union was able to quickly regain the territory, and Ukraine remained a part of the USSR until its fall in 1991.
Much of Ukraine was under the control of the Nazis, and the Ukraine Insurgent Army (UPA) attempted to break off from the USSR and become part of Germany.
Ukraine has been separated from Russia once again, and the effort to reunite the two has been faced with much difficulty.