After having withdrawn its troops from Bessarabia and Northern Bukovina in response to the June 1940 Soviet Ultimatum, Romania entered an alliance with Nazi Germany and declared war on the Soviet Union. Romanian troops entered World War II in June 1941 as part of Operation Barbarossa, under the German High Command. Following the recapturing of the territory annexed by the Soviet Union in June 1940, Romanian troops occupied Southern Ukraine all the way to the Southern Bug. However, Romania's eastern campaign ended in disaster, notably at the Battle of Stalingrad.