The American people had growing distrust of government
US provided army and weapons to go against Germany, assisting the allied power to help gaining back the liberation of France on D-Day for example.
As for Japan, after the Pearl Harbor Incident, the US joined the war, defeating the Japanese in the sea battles in pacific. US also finally lead to the withdrawal and the end of WWII by dropping atomic bombs on Hiroshima and Nagasaki.
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Immigration drastically increased in the United States during the period between the Civil War and the early 1900s. This immigration boom coupled with increased development and industrial output following the Civil War led to large economic growth in the United States as well.
Most nations were colonized by British and French, only a few countries gained independence.