The major world conflict involving Nazi Germany's invasion of Poland in 1939 was World War II. This invasion, occurring on September 1, 1939, marked the official start of the war in Europe and resulted in the swift defeat of Polish forces. The invasion's significance lay in its demonstration of aggressive blitzkrieg tactics and the subsequent declaration of war by Britain and France against Germany. The conflict would escalate globally, leading to immense human suffering and reshaping political boundaries following the war's conclusion in 1945, ultimately contributing to the onset of the Cold War.