The event that led to the initiation of World War II was the invasion of Poland by Germany on September 1, 1939. This aggressive act violated diplomatic agreements and prompted Britain and France to declare war on Germany. The invasion was part of Adolf Hitler’s broader strategy of expansion and domination over Europe, which arose from unresolved grievances stemming from the Treaty of Versailles. This conflict would evolve into a global war, involving numerous nations and resulting in widespread devastation, reshaping international relations for decades to come.