Pretty hard. The war with China was the main reason for the Pacific War (Japan needed oil), and the reason for the war with China in 1937 was the conquest of Manchuria in 1931. So the problem goes back a long ways.
One possibility is for the Manchuria incident (or an alternate version of this incident) to develop into an all-out war with a China that is even weaker due to less political unity or military-industrial might. This was before the Sino-German cooperation after all. Anyway, the results of this war could be a disaster for China, and very well force the KMT to accept the loss of Manchuria.
But Chinese nationalist sentiment will not be amused, and by the latter half of the 1930s we start seeing a rather militantly anti-Japanese administration take the reins. By 1940, seeing that the Chinese aren't simply going to behave, the Japanese navy develops their "Strike south" idea as IOTL, which they intend to carry out in order to gain a better position with which to dominate and check China indirectly.
Since the Philippines are in the way of whatever the Japanese would try to take, the US probably still goes to war.