Political map of North America & the Caribbean 14 February 1944 (America in World War II): Following the Japanese attack on Pearl Harbor, Germany declared war on the United States (German declaration of war against the United States (1941)), bringing its U-boat campaign to the East Coast (Second Happy Time). Despite initial setbacks in both the Atlantic and Pacific, the US took only six months to defeat Japan at sea, allowing it to focus on the war in Europe.