Atreus,
You're greatly overestimating US belligerence. Before Pearl Harbor, Germany had already torpedoed several US warships were and sank USS Reuben James with a loss of ~115 lives and no war resulted.
In November of 1941, the US Congress passed an extensive of the Selective Service Act by one vote.
Given US racial attitudes at the time, a truce after Japan begins 'skirmishing' with the US is totally implausible. In the OTL, FDR found it hard to maintain the 'Germany First' policy.
In the OTL the vast majority of US war production was directed to the fight against Germany. Even if you add the what was historically sent to the Pacific theater, you still won't have enough for an amphibious assualt in northwestern Europe by 1943. There simply isn't enough time to build what is necessary, train the troops required, and accomplish all the other prerequisites; i.e. beat down the Luftwaffe.
Which they'd win in less than a month. In the OTL, the USSR beat Japan inside a week. Granted, the Kwangtung Army had been raided for close to a decade for combat units and equipment for the many China, Burma, and Pacific campaigns and over 50% of the force the USSR beat in August of '45 was made up of reservists called up that month. However, the Soviets had already kicked Japan's ass in 1939, which was before they suffered through that "graduate course" in land warfare Germany sprung on them. In the OTL's 1939 Japan's armor, artillery, and other weapon were already second-rate and Japanese doctrine very poor. By this ATL's 1944 the difference would be even greater.
What happened to your truce?
With Germany knocked out early would the Manhattan Project even be finished when it was in the OTL? With the USSR occupying Manchuria and Korea very early on, they'd be in Hokkaido well before the Us got anywhere near Honshu.
Bill