...
Without a war in Europe, how does the Pacific War go? I can't see the Japanese elevating things against the French and British without a war in Europe, but presumably the Americans would continue to respond to Japanese actions and the Western Nations will continue to smuggle weaponry and supplies into China. ...
I my choice for more likely scenario is enough Japanese leaders, in and outside the government, in the military and the Zaibatsu, recognize the China war as a losing proposition. A peace is brokered and Japan retrenches for a tougher economic competition in the 1940s, and for a resurgent China becoming a military threat to Japans empire.
However if you want a Pacific war by the early 1940s heres a few possible paths. I don't consider any of them likely, tho they are possible.
Closing Haiphong.
OTL one of the motivations for the Japanese occupation of French Indo China was to close the overland trade route from the Hanoi port to the KMT territory. France was uncooperative over closing the supply route to the KMT. There were other economic incentives as well. FIC was a net rice exporter, supplying Asia wherever shortages developed. There were the lucrative rubber plantations. Tin ore was mined there. OTL FIC looked like low hanging fruit & the Japanese jumped on it, never expecting the US/British reaction to be so devastating.
Lets imagine for a moment the same problem exists with military supply and other trade to the KMT via the Red River basin undercutting Japanese efforts in China. Now consider that without a resurgent Germany the former Entente has not been revived in a Anglo/French alliance. France, Britain, the US, & others are all drafting along fat dumb and happy. In that political context could Japan pull off the same stunt they did vs the Russians 35 years earlier? Make some strong demands of the French, when negotiations seem to stall Japan makes a surprise attack on the French Asian fleet & invades FIC at several points. France lacks the ability to compete at sea with Japan. It can send its European fleet, but that looks suspiciously like a reprise of the Russian debacle of 1906. Britain and the US will flail around in confusion Italy is unlikely to support France, & how effective can French/German cooperation be at this point. From the Japanese perspective this could look like a winning situation. The French fleet is too weak, the French army in FIC is small and as we all know no match for the Japanese army in morale and skill

. As with the Russians Japan can get a quick victory and negotiate some substantial concessions from the French. What could possibly go wrong here?
End the Concessions
The whole point to Japans invasion of China was to gain maximum economic control. The foreign Concessions, the Unequal Treaties, gave the Europeans considerable economic leverage in China. If those are reduced or ended Japan gains more economic control & gains prestige among the Chinese for ending the hated Concessions. Japan can act directly as representative of its puppet Chinese government, or put the puppet government up front and back its demands with Japanese muscle. Either way the European businessmen will be howling for their governments to preserve their lucrative setup. But, in Japans leaders view the Europeans are morally and militarily weak, and the US not even worth considering. Again, what could possibly go wrong?
Bat S...t Crazy
The KMT collapses in 1942. Within a couple years the imperialist see nothing but opportunities across Pacifica and Asia. The natives are restless & the colonial empires looking shaky. Lets liberate them and create a great pan asia CoProsperity Sphere. The European empires will collapse like a table of card houses. What could go wrong? Just kick the door in...
Debt Reduction
The KMT collapses but Japan is clearly losing the peace, with control of China costing more than it brings in. The debt burden incurred in the US, and Britain can't be serviced. Japan makes demands, then in a surprise move invades the Philippines in a effort to gain some negotiating leverage. The mongrel Americans have no army to speak of, and the Navy is mostly for show, having not fought a serious war since 1863. Again, what could go wrong here?
