1.) Sanctions set on Japan as a discipline for proceeding with their war with China. This brought about a percieved oil and metal assets emergency in Japan. "Having just year and a half of oil stores" is a ton of oil, in spite of the fact that this was developed to be an enormous purpose behind war, by the Japanese aggressive government.
2.) The conviction by the Japanese military government that America was a country of "Apathetic Playboys", who wouldn't battle at any rate, yet would surrender under weight from a solid assault.
3.) American military backing for patriot China. The American airforce was at that point flying P-38 contenders against the Japanese military, apparently under the charge of the Chinese armed force. These were the "Flying Tigers".