Answer: Because Truman asked Stalin for assistance with Japan and Stalin said america did not need the USSR's help.
<span> Because the </span>United States<span> failed to </span>become<span> a member of the League of Nations. Indeed, </span>isolationism<span> would persist for a few more decades. Well During the 1920s. Then </span>American<span> foreign affairs took a back seat.</span>