Russia–Ukraine relations refers to the relations between the Russian Federation and Ukraine. Currently, the two countries are engaged in the Russo-Ukrainian War which started in 2014 following Russian annexation of Crimea from Ukraine. The modern bilateral relationship between the Russian Federation and Ukraine formally started during World War I as the former Russian Empire was going through its political reform. In 1920, the bilateral relationship between the two countries was changed as Ukraine was conquered by the Russian Red Army and Polish Army. In the 1990s, immediately upon the dissolution of the Soviet Union, of which both Soviet Russia and Soviet Ukraine had been formally founding constituent republics, bilateral relations were revived. Relations between the two countries have been hostile since the 2014 Ukrainian revolution, followed by Russia's annexation of Crimea from Ukraine, and over Russia's backing for the separatists fighters of the Donetsk People's Republic and the Luhansk People's Republic in a war that by early 2020 had killed more than 13,000 people and brought Western sanctions on Russia.