The History of the Balkans is complicated and the network of ambitions and counter-claims following the decline and expulsion of the Ottoman Empire led to numerous small conflicts and border tensions. The Catholic Hapsburg Empire was extending its influence in Bosnia and treading on Serbian sentiment. In truth the Austrians had been looking for a reason to deal with the Serbian issue for some time although the assassination of the Archduke was rather more provocation than they anticipated. The Kaiser was on holiday on his yacht when war broke out.
The network of alliances between Russia and Serbia, France and Russia, Britain and Belgium, Germany and Austria led to a domino effect escalation. However, it was the golden age of Empire and a number of these countries were actually keen to bitch slap each other to enhance their global pecking order place. All were confident it would be over "by Christmas".
Was born out of the bitter frustration of the Treaty of Versailles on the German side. It was the populist feeling in Germany that they had carried the can for a war that was not of their making. Hitler played on this resentment and his offer of restoration of German pride played well in the beer halls. It is doubtful the Nazi movement would have took off were it not for the impositions of Versailles. As part of the mythology sold by Hitler was that Jews, Communists and Liberals had betrayed Germany in 1918. He preached his polemic and thrived on conspiracy and hatred against these people. So, yes the German Nazi movement started WW2 but WW2 was really just WW1 part deux. If WW1 had played out differently and reparations against Germany less vindictive then it is unlikely the German economy would have failed in the 20s and opened the door to the fanatics and extremists of the Nazis.