The Serbian assassination of the Austrian Archduke Ferdinand.
Chat with our AI personalities
World War I had its beginnings in a serious "disagreement" between rival groups within the Austro-Hungarian Empire, the dominant group and a handful of independence-minded groups which resented imperial control. When the tensions between these groups erupted into violence with the assassination of Archduke Ferdinand (a leader among the dominant group), World War I soon erupted, as well, with various nations coming to the aid of one or the other of the groups involved in the rivalry until nearly the entire world was at war.
The submarine.
World War I wasn't the most destructive war. World War II was. World War II had over eight times as many casualties as World War I and also brought total war to a whole new level. It completely put Europe in ruins, but World War II was definitely the war to end all wars. After that came the Cold War, which was just political disagreement with words thrown around.
It turned the tide of war in Allied favour
The assassination of the duke, Franz Ferdinand started it. It turned into a world war because everyone was pointing fingers at each other