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.
This event was, however, simply the trigger that set off declarations of war. The actual causes of the war are more complicated and are still debated by historians today. Now See The Real Cause OF World War 1 And What You Must Know About World War 1 here - https://9jajointlearn..com.ng/2018/04/the-real-cause-of-world-war-1.html