WW2 did not start with the Japanese invasion of China. WW2 started with the Nazi invasion of Poland, in 1939.
Why?
Allied powers delivered an ultimatum to Hitler to cease his aggression, or to find his Germany at war. Hitler called their bluff, and lost.
It has nothing to do with my opinion of the relative value of Chinese casualties, it has to do with the time when the allies became officially embroiled in the war.
The Japanese invasion of China did little, if anything to precipitate the declaration of war. It was the attack carried out at Pearl Harbor that drew the US into the war. The actual "urban legend" is that the Japanese code was broken shortly before the attack, but due to radio silence, the whereabouts of the imperial fleet were unknown. It was merely a happy coincidence that the carriers were sent out on "maneuvers".
However, information has come to light that radio silence was not followed, and that the imperial fleet engaged in repeated communications on the lead up to the attack. Further, it seems that the code was broken months before the attack. It may be possible to declare innocence of the top brass based on some incompetence within US intelligence spheres, however, this ignores the evidence of deliberate egging on of Imperial Japan.
Personally, my view of the Japanese invasion of China was that it resulted in the atmosphere that created Mao, who was one of the most brutal dictators ever to hold office, anywhere.
