In 1939, Britain and France declared war on Nazi Germany. The war was declared in response to Hitler's invasion of Poland, Britain and France.
On September 3, 1939, German forces sunk a passenger vessel carrying over 1,000 people, and 112 of those passengers lost their lives, which angered the Allied forces. The next day, the Allied forces began dropping bombs on German ships causing mass causalities. The United States vowed to stay neutral during the war, but the country joined the Allied forces in 1941 after the attack on Pearl Harbor. World War II finally ended with the Axis' surrender in 1945.