His food was pulled to the front in 750,000 GMC trucks, and by 10,000 US-made locomotives...the list goes on.
Japan made their decision not to fuck with the USSR after Khalkhin Gol. The Soviets showed up with more men, more (and better) tanks, more airplanes, and a much better general. Despite inflicting greater casualties on the Soviets, the Japanese were nonetheless defeated, decisively. As soon as the USSR got into trouble with the Germans, they hastened to make it known that they had no desires to take Vladivostok, which freed up the Russian eastern armies to head west - but also freed up Japanese Manchurian forces to work deeper into China, and the navy in the Sea of Japan to operate against the USA.
I think in the scheme of things, the Japanese looked at what they could get from the USSR - Vladivostok and a couple islands and a bunch of Siberia - and decided it was better to gamble on more resources from the South Pacific and China. Obviously the logical choice, though still a poor choice as fighting the USA was essentially a death sentence.