World War II began with Germany's invasion of Poland on September 1, 1939, following Britain and France's declarations of war on September 3. The conflict was rooted in various factors, including the aftermath of World War I and the rise of extremist politics in Germany, particularly under the Nazi regime. Ultimately, the war led to a systematic defeat for Germany, culminating in their surrender on May 7, 1945, due to strategic failures, resource mismanagement, and Allied military success.