If he had refrained from attacking Russia and would have convinced the Japanese not to attack the US neither of those would have joined in. The only reason the US joined WWI was because public opinion swayed after the Lusitania was sunk, the only reason the US joined WWII was because of Pearl Harbor, and as Japan was allied with Germany a declaration of war against Japan also was one against Germany.
If Russia and the US would not have joined much of Europe and Asia would have been under fascist occupation for many years to come.
And thanks to the competition between the US and the Soviet Union many (a whole bunch of Nazi 'scientists' and almost all Japanese fascists) got away without punishment.
Note: Without the US joining in Nazi Germany would most likely still have fallen (the Soviets, Canada, Australia, New Zealand and many others fought them too), but I'm not sure Japan would have easily fallen if they hadn't attacked either the US or Russia.