The major global conflict that began with the invasion of Poland in 1939 was World War II. The invasion, which took place on September 1, marked Germany's aggressive expansion under Adolf Hitler's Nazi regime. Britain and France responded by declaring war on Germany within days, entering a conflict that would engulf much of the world in turmoil. The war saw widespread devastation, along with atrocities such as the Holocaust, and involved numerous nations across Europe, Asia, and beyond. The conflict would eventually reshape global politics and lead to significant changes in international relations, establishing the framework for the post-war order.