When did Austria become a republic?
First Austrian Republic
The First Austrian Republic (German: Republik Österreich) was created after the signing of the Treaty of Saint-Germain-en-Laye on 10 September 1919—the settlement after the end of World War I which ended the Habsburg rump state of Republic of German-Austria—and ended with the establishment of the Austrofascist Federal State of Austria based upon a dictatorship of Engelbert Dollfuss and the Fatherland's Front in 1934. The Republic's constitution was enacted in 1 October 1920 and amended on 7 December 1929. The republican period was increasingly marked by violent strife between those with left-wing and right-wing views, leading to the July Revolt of 1927 and the Austrian Civil War of 1934.
Austria
Much like Germany, Austria was divided into American, British, French, and Soviet zones and governed by the Allied Commission for Austria. As forecast in the Moscow Declaration in 1943, a subtle difference was seen in the treatment of Austria by the Allies. The Austrian government, consisting of Social Democrats, Conservatives, and Communists (until 1947), and residing in Vienna, which was surrounded by the Soviet zone, was recognised by the Western Allies in October 1945 after some doubts that Renner could be Stalin's puppet. Thus, the creation of a separate Western Austrian government and the division of the country was avoidable. Austria, in general, was treated as though it had been originally invaded by Germany and liberated by the Allies.