World War I (1914-1918)

World War I killed 20 million people and dismantled four empires—German, Austro-Hungarian, Ottoman, and Russian—creating a power vacuum that directly triggered World War II and subsequent regional conflicts. The punitive Versailles Treaty imposed 132 billion gold marks in reparations, fueling hyperinflation and Hitler's rise. This war's geopolitical upheaval outpaces #1 in reshaping global boundaries, as 90% of modern Middle Eastern borders emerged from its aftermath.