well..there were several things... (it's compareable to English and Scotts I think) starting with the german conferderation 1815, leading to the rivalty between the House of Habsburg (Austria) and Prussia, battle of Königsgretz 1866 ("German Question"), the dismissal of Habsburg from the German Confederation, forcing Austria into WW1, occupying Austria 1938 ("the first victim")
and in '45 it was necassary (as in 1918) to establish a border in people's heads, to create a national identity, which will prevent another reunion, another Anschluss. No "Greater Germany" should ever be established again (one of the things UK & France were most afraid of), and I guess it's better that way... we are very different people in some things (espacially humor..). maybe Bavaria should join Austria, because Bavarians and Austrians come from the same tribe...
but enough history for now.