d Volhynia with their Ukrainian population became reunited with the rest of Ukraine. The unification that Ukraine achieved for the first time in its history was a decisive event in the history of the nation.
In 1940, Romania ceded Bessarabia and northern Bukovina in response to Soviet demands. The Ukrainian SSR incorporated northern and southern districts of Bessarabia, northern Bukovina, and the Hertsa region. But it ceded the western part of the Moldavian Autonomous Soviet Socialist Republic to the newly created Moldavian Soviet Socialist Republic. All these territorial gains were internationally recognised by the Paris peace treaties of 1947.
German armies invaded the Soviet Union on June 22, 1941, thereby initiating four straight years of incessant total war. The Axis allies initially advanced against desperate but unsuccessful efforts of the Red Army. In the encirclement battle of Kiev, the city was acclaimed as a "Hero City", because the resistance by the Red Army and by the local population was fierce. More than 600,000 Soviet soldiers (or one-quarter of the Western Front) were killed or taken captive there.
Although the wide majority of Ukrainians fought alongside the Red Army and Soviet resistance, some elements of the Ukrainian nationalist underground created an anti-Soviet nationalist formation in Galicia, the Ukrainian Insurgent Army (1942) that at times engaged the Nazi forces and continued to fight the USSR in the years after the war. Using guerilla war tactics, the insurgents targeted for assassination and terror those who they perceived as representing, or cooperating at any level with, the Soviet state.
At the same time another nationalist movement fought alongside the Nazis. In total, the number of ethnic Ukrainians that fought in the ranks of the Soviet Army is estimated from 4.5 million to 7 million. The pro-Soviet partisan guerilla resistance in Ukraine is estimated to number at 47,800 from the start of occupation to
