>>51395921765% of the overall Japanese armed forces(including 80% of the entire army) were bogged down in China during the war.
The US only faced a small fraction of the Japanese military in the war.
The Chinese contribution was enormous in keeping the Japanese contained.
Even in mid 1945 over 50% of the Japanese army(about 2mn remaining capable equipped men) alone was still in China and by that point, the Japanese military was ground down and barely functioning with little heavy equipment or industry to replenish the war materiel lost in non stop allied bombing raids and anti ship attacks. Then the Russians steamrolled in and literally finished off the last surviving fully intact Japanese defensive army in China in just days(US intelligence said it would take 3 months not 11 days), the biggest factor in why Japan finally surrendered the Russians were also preparing for the Soviet invasion of north Japan using a mix of lend leased US ships and craft, domestic WW1 era ones, co opted civilian vessels and captured German ones to coincide with the planned western allied landings in south Japan on the main island of Kyushu which was due to begin in November 1945.
The US dropped the first nuke just hours after the Soviets launched August Storm, hoping to end Japanese resistance before a "Red Japan" became a reality and it got split into pieces like Germany just months before.
Russia and China are the reasons the axis alliance and the respective allied nations in both continents lost. Those two vast nations were too much to take.