What if, upon being embargoed after occupying southern Indochina, decided that while war had become more likely, *they* needed to make a show of resolve short of war.
In this case, the Japanese decide to invade and occupy Thailand. The reasons are to: 1) tit for America's tat, 2) gain control of more territory to bargain with, 3) Better position Japanese forces for an all-Southeast Asia campaign if it does come to war with the ABCD powers [air power can be projected even more quickly and powerfully to Sumatra, Burma, Malaya, Java, the Maldives and Sri Lanka from the Kra Isthmus than it can from Indochina, same with ground and naval power], and finally 4) to secure the food and labor resources of Thailand.
So the Japanese invade Thailand by land, sea and air on 1 September 1941.
How quickly can they occupy the whole country?
Will Britain or the US declare war over this action?
Will the Thai surrender and possibly switch sides, and if so, how quickly?
Assuming that neither Britain nor the US jump into war with Japan right there and then, what do the western powers do besides complain?
I would assume they would get more serious about defense, especially the British in Burma and Malaya and Singapore.
What else? The US, having frozen assets and embargoing oil, is out of economic cards to play.
Do the Americans follow their pattern developed in the Atlantic (like they did in occupying Greenland and Iceland and Caribbean bases), and occupy strategic island groups like Fiji, the Solomons and Gilberts (with British consent) or New Caledonia?
Would the Americans refuse the negotiations they took part in outright, thus leaving Japan certain it needs to start the war even earlier, in October or November.
Finally, if the Japanese occupy Thailand in early September, there is no war but rather continued buildups on all sides, concurrent with diplomacy, and then war starts on December 7-8 as in OTL, how are the early campaigns and the peak territorial extent of the Greater East Asia co-prosperity sphere effected.
Do the Americans and British make any more effective use of their time, becoming prepared enough to hold back the Japanese in one or more areas?
Does this ultimately make Sri Lanka or Assam or Yunnan more accessible to Japanese power at its peak than it was in OTL, by putting Japanese troops into Burma, and Japanese ships into the Indian Ocean more swiftly than OTL?
Does a heightened level of anxiety for Burma and Malaya result in any changes to deployments of Australian troops at home or abroad?