In what fantasy land are you living in? By 1941, Japan was locked in a stalemate and slowly getting whittled down by the sheer number of bodies China could throw at the problem. In 1944, Japan's Ichi-Go operation showcased the futility of Japanese offensives: despite Japanese strategic successes, China could fight an asymmetric guerilla war and stretch out supply lines even while the Nationalists and Communists were themselves stuck in an unsteady balance of power. By 1945, the USSR had an army large enough to rout any Japanese occupation in mainland Asia and technology that was simply superior to what was available to Japan.

Allied efforts to supply China over "The Hump" were costly and largely ineffective, delivering just 351 machine guns, 96 mountain cannon, 618 antitank rifles, 28 antitank guns, and 50 million rounds of rifle ammunition between May 1942 and September 1944 (Taylor 2009).

Did the US play a role in the Pacific Theater? Absolutely. Would the Japanese have won if the US hadn't gotten involved? I doubt it.