Hi! A major cause, and most likely the most direct cause of WWI was the assassination of Archduke Franz Ferdinand at Sarajevo on 28 June 1914. However there are a number of factors contributed to the rivalry between the Great powers that allowed war on such a wide-scale to break out. Some other causes are rivalry between powers, nationalism, militarism, crises that happened before 1914 also played a role in the start of WW1.