The Treaty of Versailles is blamed for being the spark of the war. There was a whole bunch of instability in Western Europe and this treaty for Germany at blame for WW1 so Hitler was angry and wanted to take over all of Europe, starting with his country (where he was born) Austria-Hungary
Ans: Nationalism lead people to conflict. Archduke Franz Ferdinand was assassinated due nationalism by the Black Hand, a nationalist group that intended to liberate the Serbs from the Austro-Hungarian occupation. The death of Franz Ferdinand was one of the most important factos to cause the World War I.
