In June 1941, the German high command launched Operation Barbarossa, which was highly successful. German armies, backed by units of their Axis allies, advanced far into the Soviet Union, overrunning the defending armies. The cities of Kiev and Minsk, capitols of the Ukrainian and Belarusian Soviet Socialist Republics, fell to the advancing Axis forces. Two puppet regimes, the Ukrainian Republic and the Belarusian Republic, were established in occupied Belarus and Ukraine. As they were being established, the German 8th, 11th, and 16th Armies began moving in on Moscow, with the V Panzer Corps. They were aided by the Hungarian Second Army, the Romanian Fourth Army, the 339th Croatian Infantry Regiment ("Croatian Legion"), and the Slovak First Army.