World War I was triggered by a combination of long-term factors including militarism, imperialism, and the formation of mutual defense alliances, along with the short-term spark of the assassination of Archduke Franz Ferdinand. The conflict escalated as major powers engaged in a destructive global war characterized by new military technologies, resulting in over nine million deaths and significant political upheaval. Ultimately, the war reshaped international relations and led to the collapse of several empires, with lasting lessons about the consequences of nationalism and militarism.