Hapsburg Archduke Ferdinand whose assassination is often given as the immediate cause of World War I.
Alan: There are no simple, satisfying or even coherent explanations for "what caused World War I. A retired Air Force General friend, AC, a historian of World War II, does not attempt explanation. My working hypothesis is that "World War 1" happened much like spontaneous combustion happens, without any willful intent at the outset, but once hostilities were set in motion, the war "took on a life of its own." What is certain in my mind is that World War I began the unending destabilization of The Middle East, and -- due to the harsh punishment imposed on post-War Germany -- created the resentment that led to Hitler's rise and, by extension, World War II.