The US became involved in World War II to oppose the aggression of
the Axis powers, including Japan, Germany, and Italy.
the Chinese region of Manchuria.
the Sudetenland of Czechoslovakia.
the Allied powers, including Great Britain and France.
The US joined WWII because German bombers sank the US submarine Lusitania and killed Americans in the process. Germany was very responsible of disobeying Great Britain's policy on no unrestricted submarine warfare, which is sinking merchant ships without warning.
