On this day, the provisional government of Hungary officially declares war on Germany, bringing an end to Hungary's cooperation—sometimes free, sometimes coerced—with the Axis power.
Miklas Horthy, the anticommunist regent and virtual dictator of Hungary, who had once hoped to keep his country a nonbelligerent in the war, had reluctantly aligned Hungary with Hitler in November 1940. While ideologically not fascist, Hungary had many radical right-wing elements at play in its politics, as well as a history of anti-Semitism. Those radical forces saw many common "ideals" with Nazism and believed the future lay with Germany. So though Horthy little admired Hitler personally, he felt the need to placate influential parties within his own country and protect his nation from Soviet domination.
When the Germans invaded the Soviet Union, Hitler demanded that Hungary mobilize its military against the Soviets as well. So on June 29, 1941, Hungary declared war on the USSR. In March 1942, Horthy replaced Prime Minister Lazlo Bardossy, (a political manipulator too eager to piggyback on German territorial expansion and turn on former allies for the sake of personal gains), with Miklos Kallay, who shared the regent's goal of regaining the favor of the Western—non-Soviet—Allies. Kallay was able to communicate to the Allies that Hungary was open to switching sides again should they make it to Hungary's border and offer Hungary protection from German and/or Soviet occupation.
In January 1943, the Battle of Voronezh against the USSR saw Hungary's entire 2nd Army decimated by the Soviets, rendering Hungary militarily impotent. Hitler, who learned of Kallay's sly communiques with the West, gave Horthy an ultimatum: Either cooperate fully with the German regime or suffer German occupation. Horthy chose to collaborate, which meant the suppression of left-leaning political parties and an intense persecution of Hungary's Jews, including massive deportations to Auschwitz, something Kallay, to his credit, had fought to prevent. (More than 550,000 Hungarian Jews—out of 750,000—would die during the war.)
As Soviet troops began to occupy more Hungarian territory, a desperate Horthy signed an armistice with Moscow. When the regent announced this on radio, he was kidnapped by the Germans and forced to abdicate. Ferenc Szalasi, leader of the fascist Arrow Cross Party, was made head of the country on October 15, 1944, though he was little more than a puppet of the Germans. His rule of terror, especially against Hungary's Jews, would become infamous.
Soviet troops finally liberated the bulk of Hungary from German rule in December 1944. On December 31, a Provisional National Assembly, composed of Communists loyal to the USSR, officially declared war on Germany. The Assembly would go on to sign an armistice with all the Allies in January of 1945.