Events of World War I World War I contained countless events that changed history. Unlike many other wars, World War I had many causes. Two of which draw attention are the arms race and the assassination of the archduke. The beginning of World War I then led to gas warfare as a new way of effectively killing the enemy and later the neutral US entered. The arms race, the assassination of the archduke, gas warfare, and the entrance of the United States are four important events in World War I that have clear causes and effects. Another cause of World War I is the assassination of the Archduke Franz Ferdinand. This was the main event that set off World War I. It began in June of 1994 Archduke Franz Ferdinand and his wife were leaving the Sarajevo