The Allied occupation of Austria lasted from 1945 to 1955. Austria had been regarded by Nazi Germany as a constituent part of the German state, but in 1943, during World War II, the Allied powers agreed in the Declaration of Moscow that it would be regarded as the first victim of Nazi aggression, and treated as a liberated and independent country after the war.

In the immediate aftermath of the war, Austria, like Germany, was divided into four occupation zones and jointly occupied by the United States, Soviet Union, United Kingdom and France. Vienna, like Berlin, was similarly subdivided but the central district was administered jointly by the Allied Control Council.

Whereas Germany was divided into East and West Germany in 1949, Austria remained under joint occupation of the Soviet Union until 1955; its status became a controversial subject in the Cold War until the warming of relations known as the Khrushchev Thaw. After Austrian promises of perpetual neutrality, Austria was accorded full independence on 12 May 1955 and the last occupation troops left on 25 October that year.

At the 1943 Moscow Conference, the allies had jointly decided that the German annexation of Austria in 1938 would be considered "null and void". As well, all administrative and legal measures since 1938 would be ignored. The conference declared the intent to create a free and independent Austria after the war, but also stated that Austria had a responsibility for "participation in the war at the side of Hitlerite Germany" which could not be evaded.

On 29 March 1945 the Soviet commander Fyodor Tolbukhin's troops crossed the former Austrian border at Klostermarienberg in Burgenland. On 3 April, at the beginning of the Vienna Offensive, the Austrian politician Karl Renner, then living in southern Lower Austria, established contact with the Soviets. Joseph Stalin had already established a would-be future Austrian cabinet from the country's communists in exile, but Tolbukhin's telegram changed Stalin's mind in favor of Renner.

...
Wikipedia