Why did the united states join the united nations?

      The impetus to establish the United Nations stemmed in large part from the inability of its predecessor, the League of Nations, to prevent the outbreak of the Second World War.  Despite Germany’s occupation of a number of European states, and the League’s failure to stop other serious international transgressions in the 1930s, such as Japan’s invasion of Manchuria, many international leaders remained committed to the League’s ideals.  Once World War II began, President Franklin D. Roosevelt determined that U.S. leadership was essential for the creation of another international organization aimed at preserving peace, and his administration engaged in international diplomacy in pursuit of that goal.  He also worked to build domestic support for the concept of the United Nations.  After Roosevelt’s death, President Harry S Truman also assumed the important task of maintaining support for the United Nations and worked through complicated international problems, particularly with the Soviet Union, to make the founding of the new organization possible.  After nearly four years of planning, the international community finally established the United Nations in the spring of 1945.    

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answer; /// i believe that the correct answer is (d)the huang he river valley;