What event caused the beginning of world war

WW1: The first world war began in August 1914. It was directly triggered by the assassination of the Austrian archduke, Franz Ferdinand and his wife, on 28th June 1914 by Bosnian revolutionary, Gavrilo Princip.

WW2: the rise of Italian fascism in the 1920s. Japanese militarism and invasions of China in the 1930s. the political takeover in 1933 of Germany by Hitler and his Nazi Party and its aggressive foreign policy starting in 1936.
 
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Although a variety of different factors caused World War Two, the main event that sparked it all was Germany's invasion of Poland in September 1939. The Treaty of Versailles, signed at the end of WW1, placed some very harsh restrictions on Germany, which created a feeling of resentment among the Germans.