It couldn't have been settled peacefully by the United States because by the time we entered it was beyond our capacity for diplomacy. Hitler was wholly unwilling to negotiate. 
Hitler didn't want the war in the first place.
He didn't want to fight the British - who he respected, and the war wasn't his idea at all. Hitler offered to resign if that would assist the commencement of peace negotiations, but the efforts met with no response.
Churchill had actually forbidden his diplomatic corps to entertain or forward any German peace proposals, even via neutral countries.
You know Rudolf Hess? He went to Britain to stop WWII from happeneing. He flew a plane to Britain to take Hitler's peace proposals directly to the king, via the Duke.
He was attacked by anti-aircraft fire and jumped out in a parachute - and broke his ankle.
That one break may have been the deciding factor of WWII.
He was found, jailed, and the war did not stop.
By no fault of his own. Abe did everything within his power to restore the union, but none of his actions led to the war. 
What made him a good president?
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"If you worship your enemy, you are defeated.
If you adopt your enemy's religion you are enslaved.
If you breed with your enemy you are destroyed."
-Polydoros