If the US hadn't joined after the attack of pearl harbor (or if the attack never happened as it happened much sooner then Hitler wanted) then in all likelihood Japan would have conquered all of the eastern side of Asia and Australia. If the US still refused to enter the war, the Japanese forces would have fought Russia coming from the East while Germany came from the West, most likely resulting in a loss for Russia.
If the US never joins, Germany and Japan had a good chance of controlling all of Europe and Asia, leaving the US isolated. This was actually Hitler's gameplan but Hirohito bombed pearl harbor too early.
