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The act which is considered to have triggered the succession of events which led to war was the 28 June 1914 assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by Gavrilo Princip, a Bosnian Serb citizen of Austria-Hungary and member of the Young Bosnia. The retaliation by Austria-Hungary against the Kingdom of Serbia activated a series of alliances that set off a chain reaction of war declarations. Within a month, much of Europe was in a state of open warfare.
What I'm wondering is, would WWI still have happened even if the assassination of Archduke Franz Ferdinand had never taken place?
Yes I believe it would of happened sooner or later because of the tensions that existed around that time due to the alliances that had been formed. Now if there hadn't been the two sets of alliances then I think it could of been different. Perhaps a local war between Serbia and Austria/Hungary but even without the Alliances I think this would of drawn in Russia (and by consequence Germany and perhaps the Ottoman Empire) and so and the end of the day the result would of been similar. However I think it could of been a solely central Europe war depending on how the British, French and Italians reacted. If the Italians had reacted then I still think that the British and French could of avoided participation for at least a couple of years but eventually something would of triggered their involvement. At the end of the day I would say it would be a mainly European war that would be unlikely too involve the United States or most other parts of the world except for the colonies of the countries participating.