With all these Hitler Joins threads I thought I'd give it a try. What if Hitler joins the Austrian Army and just for shits and giggles lets say he fights (and survives) the Italian front!
If Hitler survive, the biggest difference is that he won't live in Germany after the war, which mean we have a Hitler whom live in Vienna, likely he will go into politic, but he's unlikely to be anything other than a minor player in Austria.
Even if his politics succeeded wildly in Austria - as they admittedly did in Germany - and he became Austrian chancellor, he'd have an extremely hard time getting anything done on the exterior front. Austria is just too small and powerless. He'd have to play along the conservative front that would appear in Germany and the best he could hope for is to facilitate an Anshluss in exchange for some position in Germany's ruling party. But even he, from the position of an outsider coming in 'for services rendered' could pull himself into Fuhrerdom. He'd stay a secondary figure at best, seen maybe as instrumental to reunification.
And this is a best case scenario for him - doesn't take much to make him spend his life as a nobody in Vienna. I suppose it depends on whether destiny beats butterflies and by how much. Each author is given considerable leeway when it comes to this.
I would be that hard for him to get into power (Austria was kind of Neo-Fascist) but as others have said he'd be much too weak to influence the world greatly,maybe an Austro-Italian War in the early Thirties over the Tyrol.