The thing is, Kuomintang China actually doing better in crushing the Warlords, preferably because of German advisors help during the Sino-German cooperation parts, would certainly do the trick.
Also this China partially industrializes and actively engaged Japan on Manchurian front (nothing big initially, just skirmishes and tit for tat attacks).
Then Hitler invaded Poland, sparking WW2, and made overtures to China to attack Indochina to made diversions for France.
Chiang still mulls things up, but Fall of France happened and China thought Axis is on the path of victory, decides to "liberating" French Indochina and South East Asia. Unlike Japan IOTL though, they were sincere in their liberation (since they didn't hope to hold them) and just put leaderships into the habds of grateful native leaders there, because China's real aim is to wheel North after fulfilling their promise to Germany, and smash through Manchuria and Korea to reunite China for good.
Just in time with Japan attacking China to alleviate pressure on their Western European allies.
US being explicitly both pro-china yet also pro Western Europe, decided that they'll go full neutral. China managed to get Manchuria but stuck in Korea...
And then Soviet Union attacked China, sparked Germany to try Barbarossa earlier, and now US supporting Axis more...
Suddenly, Britain, warry with US aid to Axis countries... Decides of surprise attack on Norfolk. The Day of Infamy is done ITTL By THE PERFIDIOUS ALBION.
...
Long Story Short, in alternate conference discussing Post-War Order, FDR sticking like a sore thumb between Hitler, Mussolini, and Chiang, all while loathing how he must smile, shook hands, and sit at Hitler's side.