This statement refers to the American entry into World War II. Prior to America's involvement in the war, the country had pursued a policy of isolationism. However, the attack on Pearl Harbor by Japan in December 1941 led to the United States declaring war and actively participating in the conflict alongside its allies. This signaled to other world powers, particularly Nazi Germany and Imperial Japan, that the United States would be a formidable and decisive force in the war, ultimately playing a key role in the Allied victory. The US' increased involvement also demonstrated its commitment to shaping the post-war world order and maintaining a leadership role in global affairs.
