If you know your history, you might recall that Germany and Austria had kind of been engaged in a struggle for hegemony over the culturally German population of Europe ever since the days of Prussia. If what you're trying to imply is that there was no difference between Austria-Hungary and Germany, there obviously was.
And yes, Hitler was born in Austria-Hungary (granted it was apparently a town right on the border). With a bit of further research, it looks like he was an Austrian citizen until 1925, when he renounced it but didn't apply for German citizenship, which caused him some problems with running for office and the risk of getting deported from Germany. He fought in the Bavarian army as an Austrian citizen. His grandmother was also born in Austria. What was your point?
Hitler was more or less directly responsible for World War 2, however.