This was an Austrian effort by a group of army personnel opposed to the Nazi regime to bring about the peaceful surrender of the city of Vienna to the advancing Soviet army (5/8 April 1945).
After the Soviet forces of Marshal Sovetskogo Soyuza Fyedor I. Tolbukhin’s 3rd Ukrainian Front and Marshal Sovetskogo Soyuza Rodion Ya. Malinovsky’s 2nd Ukrainian Front had defeated the German ‘Frühlingserwachen’ offensive in Hungary between Lake Balaton and the Danube river, they advanced toward Austria, at the time part of Germany, crossed the frontier on 29 March at Klostermarienberg, and approached Vienna on 6 April in the 'Vienna Strategic Offensive Operation'.
Adolf Hitler ordained that Vienna was to be held regardless of cost to the civil population and the area’s physical infrastructure which, in the case of the latter, was to be destroyed rather than allowed to fall in Soviet hands. To this end Vienna’s railway stations, trains, bridges, signal towers, oil storage tanks, electrical and gas works, technical equipment and food supplies, among other vital installations, were prepared for demolition.
In an effort to prevent this, a resistance group of Austrian officers contacted the leadership of the Soviet forces approaching the city. The leader of this Austrian group was Major Carl Szokoll, who had been the Viennese contract of the German group which had tried to assassinate Hitler on 20 July 1944, but whose involvement in the 'July plot' had remained undetected. Then, with the support and aid of the O5 resistance group in Austria, Szokoll had worked within the Wehrkreiskommando XVII's staff to create another resistance movement.
On 2 April 1945 Oberfeldwebel Ferdinand Käs and Obergefreiter John Reif managed to establish contact with the command of Tolbukhin’s 3rd Ukrainian Front at Hochwolkersdorf. The Soviets expressed a natural interest in the thinking of the resistance group, which provided the Soviets with information about the strength and deployment of the German forces and suggested a way in which a weakness in the German defence could be exploited to facilitate the forthcoming Soviet drive to take the Austrian capital. What the resistance proposed was that the Soviets should encircle the western side of Vienna in the area where the defences were weakest as the Germans expected the primary Soviet assault to be delivered from the east. Plans were also made for the way in which the city’s civilians could be spared as much of the fighting and destruction as possible. Another arrangement was that once the Soviet assault had begun, in the following day the resistance group would fire flares into the sky as confirmation that the city’s civilians had risen against the Germans.
Shortly after Käs and Reif had returned to Vienna on 4 April, Major Karl Biedermann, commander of the army patrol organisation in greater Vienna, was betrayed and arrested. As his part of 'Radetzky' (ii), which was now partially compromised, Biedermann was to have secured the city’s bridges.
Even so, during the night of 5/6 April, 'Radetzky' was launched, and one of its first objectives was the seizure of the Bisamberg radio station in the northern suburbs, but the plan was discovered and Hauptmann Alfred Huth and Oberleutnant Rudolf Raschke were arrested, sentenced by an SS drumhead court and, on 8 April hanged from street lights in Floridsdorf, as was Biedermann. Szokoll received sufficient warning to escape arrest and flee to the headquarters of the 9th Guards Army in Purkersdorf, where he informed the Soviets of the failure of 'Radetzky' (ii).
Although 'Radetzky' (ii) was partially successful in preventing the Germans' planned demolitions in Vienna, and the encirclement of the city was successful, the fighting lasted from 6 to 13 April and resulted in the deaths of some 19,000 German and 18,000 Soviet soldiers.