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There were many factors which led up to World War I. It was primarily a conflict over disputed land rights and territorial boundaries. The assassination was simply the match that lit the tinderbox. There is even speculation that the two "terrorists" who performed the assassination were simply paid innocents who didn't even realize what they were doing. They were hired simply to get ignite bad feelings on all sides and make war more inevitable.
The assassination of Archduke Franz Ferdinand was the catalyst for World War I, but the principle cause was in reality the complicated structure of alliances and rivalries between European powers which preceded the war. Franz Ferdinand's assassination was merely the event that set off the already existent tensions. Due to the alliances and rivalries between the powers, had the Archduke not been assassinated, it is highly likely another event would have eventually triggered a war anyway.
The assassination was at most a flash point in what was already a march to war. Nationalism was the main cause of WWI in hindsight, and the entire event was almost purely a political argument between several ruling monarchs and governments trying to assert themselves on the greater world stage.
No, the assassination of Archduke Franz Ferdinand should not be considered the main cause of World War I. There is a difference between cause and catalyst. The main cause of World War 1 was worsening relationships between various nations and European monarchs; war was inevitable. The Archduke's murder was simply the catalyst needed for hostilities to commence.
I do not believe the assassination of Archduke Franz Ferdinand was the main cause of World War I, but it was the first event in a line of many that led up to the war. The importance of this event should not be down played but saying that it is the main cause is not accurate.