The global conflict that began with the invasion of Poland by Germany in 1939 was World War II. This war was characterized by widespread atrocities, including the Holocaust, and involved numerous countries across the globe, including major powers like the United States, the Soviet Union, and the United Kingdom. The invasion of Poland on September 1, 1939, marked Germany's aggressive expansion under Adolf Hitler and led Britain and France to declare war on Germany shortly thereafter. Over the course of six years, WWII resulted in significant geopolitical changes, the emergence of the United States and the Soviet Union as superpowers, and vast human and economic losses.