Despite speaking the same language, an Austrian national identity was promoted and established which has prevented any attempts to unite with Germany since. Why didn't the Allies try to do this with West and East Germany to try and prevent any attempts to reunify the two countries? Couldn't East Germany be Prussia, for example? Or was it just considered impossible that Communism would ever fall in East Germany and cause the unification of the two?
The allies (NATO) after WW2 wanted a strong reunited Germany to help fight the spread of communism from the east. Similarly, the Soviets saw the possibility of a United communist Germany as desirable as a good ally against the west. This is because Germany, being the major country of Central Europe is strategically very important, and was known as a key industrial nation, with a lot of expertise and resources even after WW2.
The attitude of both nato and the soviets can be seen in their treatment of Germany after the war. Both were remarkably lenient on the population after the initial detention and analysis of suspected nazis. Both allowed their part of Germany some form of self governance within 10 years. And both allowed Germany to rearm.