There were no compelling reasons for the US to enter before it did.
Germany, Italy and Japan had not declared war on the United States.
Until 1941, no US territories interests had been attacked.
Germany's unrestricted warfare did not include ships traveling between or just outside of US ports.
No large groupings of US citizens had been killed or injured until 1941
The wars in Europe and Asia did little to interfere with US trade.
With the United States slowly exiting The Great Depression war would have been a hard sell with the above; without them, which is what happened, it was almost impossible. In fact, had the Japanese not attacked Pearl Harbor and had the Germans worked out some compensation for the sinking of the destroyer USS Reuben James six weeks earlier, it's difficult to see how the United States could have justified enter the war until at least 1943 at the earliest.
Even then The United States almost certainly would have been in conflict with Japan alone, as Germany's declaration of war was the only thing that allowed the US the legal sanction to enter the European Theater of conflict. The US had no defense alliances with either France or Great Britain that obligated it to come to their defense if attacked.
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