After being granted part of southern Czechoslovakia and Subcarpathia by the Germans and Italians in the First Vienna Award of 1938, and then northern Transylvania in the Second Vienna Award of 1940, Hungary participated in their first military maneuvers on the side of the Axis powers in 1941. Thus, the Hungarian army was part of the invasion of Yugoslavia, gaining some more territory and joining the Axis powers in the process. On 22 June 1941, Germany invaded the Soviet Union in Operation Barbarossa. Hungary joined the German effort and declared war on the Soviet Union on 26 June, and entered World War II on the side of the Axis. In late 1941, the Hungarian troops on the Eastern Front experienced success at the Battle of Uman. By 1943, after the Hungarian Second Army suffered extremely heavy losses at the river Don, the Hungarian government sought to negotiate a surrender with the Allies. On 19 March 1944, as a result of this duplicity, German troops occupied Hungary in what was known as Operation Margarethe. By then it was clear that Hungarian politics would be suppressed according to Hitler's intention to hold the country in the war on the side of the Nazi Third Reich because of its strategic location. On 15 October 1944, Horthy made a token effort to disengage Hungary from the war. The Germans launched Operation Panzerfaust and Horthy's regime was replaced by a fascist puppet government under the pro-German Arrow Cross leader Ferenc Szálasi, thus effectively ending the possibility for independent actions in the war. However, the form of government was only changed to a republic two years later.