Hungary formally entered World War II as an Axis Power on 26 June 1941, declaring war on the Soviet Union after unidentified planes bombed Kassa, Munkács, and Rahó. Hungarian troops fought on the Eastern Front for two years. Despite some early successes, the Hungarian government began seeking a secret peace pact with the Allies after the Second Army suffered catastrophic losses at the River Don in January 1943. Learning of the planned defection, German troops occupied Hungary on 19 March 1944 to guarantee Horthy's compliance. In October, as the Soviet front approached and the Hungarian government made further efforts to disengage from the war, German troops ousted Horthy and installed a puppet government under Szálasi's fascist Arrow Cross Party. Szálasi pledged all the country's capabilities in service of the German war machine. By October 1944, the Soviets had reached the river Tisza, and despite some losses, succeeded in encircling and besieging Budapest in December.