Perhaps you should point your Google search engine at "League of Nations" to fill in the blanks of your knowledge about the UN prior to World War 2.  It didnt exist until after the end of the war.  (If my memory serves, it was a result of the Potsdam meeting of Stalin, Roosevelt and Churchill)

Any idea that the UN has been a continuously effective body is a fantasy.  Plenty of nations have ignored the UN, not just the U.S.

Regards,

Andy