alternatehistory.com

Asia first, also known as Japan first, was the key element of the grand strategy agreed upon by the United States and the United Kingdom during World War II after the attack on Pearl Harbor. According to this policy, the United States and the United Kingdom would use the preponderance of their resources to subdue Japan in Asia first. Simultaneously, they would fight a holding action against Germany in the Atlantic , using fewer resources. After the defeat of Japan—considered the greatest threat to the USA and British Asia —all Allied forces could be concentrated against Germany .

At the December 1941 Arcadia Conference between President Franklin Roosevelt and Prime Minister Winston Churchill in Washington, shortly after the United States entered the War, the decision for the "Asia First" strategy was affirmed. It was also discussed what to do with the Victor of the Franco-Thai Conflict, Plaek Phibunsongkhram, the Prime Minister of the former Kingdom of Thailand and now Empire of Thailand, who had "recovered" all of the territorial losses from French (which meant German) Indochina.
Roosevelt seem to have suggested to leave the country be, one enemy more could not help but only hinder the war effort, especially since the Middle East Campaign wasn't still over yet. **

*This TL Wikipedia. I just switched the countries.

** Plaek Phibunsongkhram/Thailand War Against Indochina is much more successful, they don't join the Co-Prosperity Sphere.