In late Sep 1944, after the conquest of Romania and Yugoslavia had been secured, Soviet troops began to march across the southern border of Hungary. Secretly, some Hungarian leaders began dispatching envoys to the Soviet Union to negotiate an armistice, but the negotiations would take time to complete. Meanwhile, German and Hungarian forces made a push against the Soviets, striking in the area of Debrecen, Hungary’s second-largest city, inflicting 25,000 Soviet casualties and destroying 600 tanks. In response, the Soviet 2nd Ukrainian Front, 3rd Ukrainian Front, and 4th Ukrainian Front commenced the Budapest Strategic Offensive Operation on 30 Oct 1944, pushing the Axis lines of defense back into the southern suburbs of Budapest by 2 Nov. On 9 Dec, Soviet troops reached the Danube River north of Budapest, and by 26 Dec Budapest had been surrounded. On 31 Dec 1944, the Hungarian Provisional Government announced that Hungary would switch sides, declaring war on Germany. On 18 Jan 1945, the Germans attempted an attack from the outside to break the siege, but it would fail; on 10 Feb, the remaining 16,000 German troops made an attempt to break out from the inside, and it would fail and would be met with extremely high casualties. Soviet troops claimed victory at Budapest on 13 Feb, thus ending the Budapest Strategic Offensive Operation although the Soviets would not complete the conquest of all of Hungary until early Apr 1945.
Soviet Marshal Rodion Malinovsky permitted, as did other top-level Soviet generals, gave his victorious troops the permission to plunder as a reward to their hard fighting. What ensued was weeks of looting, rape, and murder. Swiss diplomats stationed in Budapest, after returning to their home country, reported in May 1945 that women ages ranging from 10 to 70 were brutally raped, while virtually anything of value that could be carried away were looted, and everything that could not be carried away such as furniture and pieces of art were smashed or burned. Even embassies of neutral countries did not escape harm; the Swiss embassy, for example, saw one of its officials nearly strangled to death as the safe he was guarding, containing millions of US Dollars worth of Swiss and Hungarian currency, was emptied by Soviet soldiers.
On 6 Mar 1945, the German 2nd Panzer Army and 6th SS Panzer Army launched Operation Spring Awakening (Unternehmen Frühlingserwachen) in an attempt to either recapture the oil-producing regions in the Lake Balaton area or to destroy the oil facilities to prevent Soviet use. The German offensive would be repulsed by the Soviets.