The Vienna Offensive was launched by the Soviet 3rd Ukrainian Front in order to capture Vienna, Austria. The offensive lasted from April 2 to April 13, 1945. The city of Vienna was surrounded and under siege for most of the offensive.
Previous agreements that Stalin reached with the Western Allies prior to April 1945 concerned the relative postwar political influence of each party in much of Eastern and Central Europe; however, these agreements said virtually nothing about the fate of Austria. Stalin thus decided to postpone his offensive towards Berlin - for which the Soviets were ready as early as February - and secure both the flanks for that offensive, and the Austrian territory, a valuable bargaining chips for subsequent postwar negotiations with his Allies.
After the failure of Operation Frühlingserwachen, Sepp Dietrich's 6th SS Panzer Army retreated in stages to the Vienna area. The Germans desperately prepared defensive positions in an attempt to guard the city against the fast arriving Soviets.
During the spring of 1945, the advance of Soviet General Fyodor Tolbukhin's 3rd Ukrainian Front through western Hungary gathered momentum on both sides of the Danube.
On March 30, the advancing Soviets forced the Hron River, forced the Nitra River, and, after they took Sopron and Nagykanizsa, crossed the border between Hungary and Austria. Tolbukhin was now ready to advance into Austria and take Vienna.
On April 2, 1945, Vienna Radio denied that the Austrian capital has been declared an open city. On the same day, Soviet troops approached Vienna from the south after they over-ran Wiener Neustadt, Eisenstadt, Neunkirchen, and Gloggnitz. Baden and Bratislava were overrun on April 4.
After arriving in the Vienna area, the armies of the Soviet 3rd Ukrainian Front surrounded, besieged, and attacked the city. Involved in this action were the Soviet 4th Guards Army, the Soviet 6th Guards Tank Army, the Soviet 9th Guards Army, and the Soviet 46th Army. The O-5, a group of Austrians led by Carl Szokoll and wanting to spare Vienna destruction, actively attempted to sabotage the German defense and aid the entry of the Red Army.
The only major German force facing the Soviet attackers was the German II SS Panzer Corps of the 6th SS Panzer Army, along with ad-hoc forces made up of garrison and anti-aircraft units. Declared a defensive region, Vienna's defense was commanded by General Rudolf von Bünau, with the II SS Panzer Corps units under the command of SS General Wilhelm Bittrich. The battle for the Austrian capital was characterized in some cases by fierce urban combat, but there were also parts of the city the Soviets advanced into with little opposition. Defending in the Prater Park was the 6th Panzer Division, along the south side of the city were the 2nd and 3rd SS Panzer Divisions, and in the north was the Führer-Grenadier Division. The Soviets assaulted into Vienna's eastern and southern suburbs with the 4th Guards Army and part of the 9th Guards Army. The German defenders kept the Soviets out of the city’s southern suburbs until April 7. However, after successfully achieving several footholds in the southern suburbs, the Soviets then moved into the western suburbs of the city on April 8 with the 6th Guards Tank Army and the bulk of the 9th Guards Army. The western suburbs were especially important to the Soviets because they included Vienna's main railway station. The Soviet success in the western suburbs was followed quickly by infiltration of the eastern and northern suburbs later the same day. North of the Danube River, the 46th Army pushed westward through Vienna's northern suburbs. Central Vienna was now cut off from the rest of Austria. By April 9, the Soviet troops began to infiltrate the center of the city, but the senseless street fighting continued for several days more. On the night of April 11, the 4th Guards Army stormed the Danube canals, with the 20th Guards Rifle Corps and 1st Mechanized Corps moving on the Reichsbrücke Bridge. In a coup de main on April 13, the Danube Flotilla landed troops of the 80th Guards Rifle Division and 7th Guards Airborne Division on both sides of the bridge, cutting demolition cables and securing the bridge. However, other important bridges were destroyed. Vienna finally fell when the last defenders in the city surrendered on the same day. Bittrich's II SS Panzer Corps, however, pulled out to the west on the evening of April 13 to avoid encirclement. The same day, the 46th Army took Essling and the Danube Flotilla landed naval infantry up the river by Klosterneuburg. While the street fighting was still intensifying in the southern and western suburbs of Vienna on April 8, other troops of the 3rd Ukrainian Front by-passed Vienna altogether and advanced on Linz and Graz.
By April 15, 1945, armies of the Soviet 3rd Ukrainian Front pushed even further into Austria. The completely exhausted remnants of what had been the 6th SS Panzer Army were forced to flee to the area between Vienna and Linz. Just behind the retreating Germans were elements of the Soviet 9th Guards Army and the Soviet 46th Army. The Soviet 26th Army and the Soviet 27th Army advanced towards the area north of Graz just behind the retreating German 6th Army. The Soviet 57th Army and the Bulgarian 1st Army advanced towards the area south of Graz (near Maribor) just behind the retreating German 2nd Panzer Army. None of these German armies was in any shape to do more than temporarily stall the advancing Soviet forces. Some of Vienna's finest buildings lay in ruins after the battle. There was no water, electricity, or gas - and bands of people, both foreigners and Austrians, plundered and assaulted the hapless residents in the absence of a police force. While the Soviet assault forces generally behaved well, the second wave of Soviet troops to arrive in the city were badly disciplined, looting and raping in a several-week orgy of violence that has been compared to the worst aspects of the Thirty Years War. Like Bittrich, General von Bünau left Vienna before it fell to avoid capture by the Soviets. From April 16, 1945 until the war's end he led Generalkommando von Bünau, surrendering to the Americans on VE Day. Von Bünau was held as a POW until April 1947. Bittrich also surrendered to U.S. forces and was held as a prisoner by the Allies until 1954. Fyodor Tolbukhin went on to command the Soviet Southern Group of Forces and the Transcaucasian Military District prior to his untimely death in 1949, reportedly from heart problems.