1991 - Ukraine becomes independent country.

Prior to that, Ukrainian territory had either been a territory of Russia or a vassal Soviet to the USSR in Moscow since the late 15th Century.

Both Russia and Ukraine trace their classical history back to Kievan Rus, a slavic government based in Kiev in 9th to 13th Centuries.

A key milestone is the definition of Ukranian nationalism is the Ukranian Famine http://www.unitedhumanrights.org/genocide/ukraine_famine.htm which was exasperated by the Moscow government.

This is a very broad question, and very difficult to answer/summerize given that Ukraine and Russia have a multi-centurial relationship. When looking at the history of Ukraine in relation to its ties with Russia, one mustn't limit their scope to just Russia. Timothy Snyder's book The Reconstruction of Nations: Poland, Ukraine, Lithuania, Belarus, 1569-1999 is an exemplary work that looks at the ever-shifting dynamics of these closely entwined neighbors. When looking at this part of the world on a map, one must take into account the ever-changing border lines that inevitably alter the cultural, religious, linguistic and political makeup of the area. For instance, a Cossack rebellion in Ukraine, actuated in hopes of establishing an autonomous Ukrainian state, prompted the Russo-Polish War of 1654-67 resulting in Ukraine being divided between the two governments. Russia seizing the left-bank, while Poland maintained the right, until that was also seized by Russia in 1772,1793, and 1795. Possession of the right-bank also created a bridge westward, which during the following centuries would prove vital during Russia's attempts at Westernization and Enlightenment.

In the mid nineteenth century there were a series of peasant uprisings in Russia and Ukraine. This nationalist fervor was sparked by the Crimean War, which Snyder states was a critical moment that "[marked] not only the change in Russian attitudes toward Ukraine, but a change in Ukrainian attitudes towards Russia" (Snyder 121). In 1861, serfdom was abolished in the Russian empire. This included Russian territory in Ukraine, though most Ukrainian peasants probably didn't realize this. The peasants of Ukraine were of Ukrainian descent. The gentry and their landlords were Polish. In fact, Kiev was primarily Polish. Thus, Russians thought that the peasant revolts in Ukraine were of Polish machination. And the Polish gentry were indeed, according to Snyder, trying to "take politics to the Ukrainian peasantry"(121). A similar occurrence was taking place in Russia. The Russian nobility or intelligentsia, in an attempt to free themselves of their benighted disposition, went to the peasants to educate them on their inherent misfortune. In 1863, The Valuev Decree prohibited the Ukrainian language. A maneuver that not only disheartens a people by trying to erase their identity, but is meant to cement a foreign force's dominance. Fast forward to 1918. WWI is coming to an end. Russia is still in the wake of their revolution, their own revolt against Russia's antiquated monarchy. And Ukraine declares their independence and forms the Ukrainian People's Republic. Though the [then] Soviet Union wouldn't recognize their independence until 1991.

This is barely even the tip of the iceberg. I realize that this response is incredibly deficient, even for a summary, but it helps illustrate how heavily Ukraine’s relationship with Russia, and vice versa, relies on the Commonwealth of Poland-Lithuania and how that whole area is so closely related.

Lewitter, L. R. "Peter the Great, Poland, and the Westernization of Russia." Journal of the History of Ideas 19, no. 4 (1958): 493-506.

Marc Raeff. Origins of the Russian Intelligentsia. New York: Harcourt, Brace & World, Inc., 1966.

Snyder, Timothy. The Reconstruction of Nations: Poland, Ukraine, Lithuania, Belarus, 1569-1999. New Haven: Yale University, 2003.

Edit: As pointed out by /r/orthoxerox, I got the left and right banks mixed up and edited accordingly. Under the Treaty of Andrusovo, Russia gained the left-bank in 1667. They wouldn't seize the right-bank from the Polish-Lithuanian Commonwealth for another century. The left-bank, however, should not be confused with Eastern Ukraine, as these terms refer to Ukraine during different periods of time.