From: Third rock from the Sun.
Tripartite Pact: This was a purely defensive pact. It was not required of one signatory to aid another if the signatory were the aggressor. If this were not true then Japan and Western Europe would have been at war in 1939.
Soviet–Japanese Neutrality Pact: Signed to release Soviet and Japanese force requirements on their northern borders. It benefited both parties and was well known by the world. Stalin even saw the Japanese delegation off at the train station.
While the U.S. was aiding the U.K. as much as possible and pushing the envelope of neutrality the U.S. could not declare war on Germany without the approval of Congress. Congress was not likely to do that.
When Japan attacked Pearl Harbor Roosevelt's advisors were at wits end. The U.S. public wanted retribution against Japan and the U.S. government was going to be forced to direct all resources against them in the Pacific thus watering down any help they could give the U.K. But when Hitler, almost in an offhand manner, declared war on the U.S. it was a great relief for all nations fighting against Germany. They knew, and some expressed this, that the war was now won. It was simply a matter of time.
Hitler's advisors pressed him to not declare war against the U.S. Hitler felt it would make little difference. What if Hitler had listened to his advisors? What if he had NOT declared war on the U.S.? What if the U.S. would be left fighting only Japan and was neutral, in whatever sense, towards Germany? The U.S. pressed Finland to cease offensive operations against the USSR and Finland publicly rejected their demands but privately agreed agreed. On 5 November President Ryti of Finland wrote Mannerheim asking to suspend offensive actions against the Murmansk Railway. He agreed and Finnish offensive operations were ceased against the Soviet Union.
So, if Hitler had not declared war on the U.S. a few days after Pearl Harbor and the U.S. was restricted in their material help to the U.K. and U.S.S.R. and was unable to provide any manpower at all what course would the war have taken? Would the U.S. have still declared war on Germany after defeating Japan? Or before defeating Japan? Or not declared at all? And think about this. If Germany had a free hand in Europe without direct U.S. involvement for even a two or three more years what would have been the effect in the USSR? There certainly would have been less troops needed in the West. Fewer U.S. tanks to the U.K. in North Africa. No invasion of North Africa by the U.S. in 1942 or Sicily. Italy would have been kept in the war much longer. Some very interesting stuff to think about.