Japanese-Americans were the largest Asian-American group, followed by Chinese-Americans and Filipino-Americans, with far fewer Korean-Americans and Indian-Americans, and apparently under 1000 other Asian-Americans. From the 1940 census, the numbers of these groups (including non-citizens) in thousands were:
| Ethnicity | Total | US states | Hawaii |
|---|---|---|---|
| Japanese | 287 | 127 | 160 |
| Chinese | 107 | 78 | 29 |
| Filipino | ? | 46 | ? |
| Korean | 7 | 2 | 5 |
| Indian | ? | 2 | ? |
At the time, Hawaii and Alaska were territories rather than states, and their populations weren't included in the "US states" total. Most of the people of Japanese, Chinese, and Korean origin or descent lived in either Hawaii or California. The number of Asian-Americans is only a small fraction of the US population, largely due to restrictions on immigration from Asia, starting with the Chinese Exclusion Act of 1882 and continuing with the Immigration Act of 1917 (which restricted immigration from Asia) and the Immigration Act of 1924 (which included the Asian Exclusion Act which, as one might guess from the name, essentially banned (almost all) immigration from Asia).
Since the Philippines was a US territory, Filipinos were US nationals; not citizens, but still able to move to the USA relatively freely (e.g., unaffected by the Immigration Act of 1917). Most Filipino going to the US went to Hawaii and California to work in agriculture, while many went to Washington state (the timber industry and agriculture) and Alaska (salmon canning) to work. Compared to other Asian-American populations, a much greater fraction of Filipino-Americans were men who went to work in the US, sending money home to their families, and intending to return to the Philippines. Filipino immigration was almost completely stopped by the the Philippine Independence Act of 1934.
Continuing to WWII service, again we need to note that Filipinos were a special case in two ways. First, the Philippine Division was a US Army division stationed in the Philippines. This was a segregated unit, consisting of two regiments of Philippine Scouts (the 45th and 57th Infantry Regiments) with Filipino enlisted personnel, and one white division, the 31st Infantry Regiment (which had been formed in the Philippines in 1916, and based there until its destruction during WWII; it was reformed in Korea in 1946). The enlisted men in these regiments were mostly Filipino rather than Filipino-American, but some Filipino-Americans served in them - it was one of their few opportunities to serve in the US military (other than as stewards in the US Navy). In 1941, the Philippine Scouts also included the 26th Cavalry Regiment (to become famous for the last US cavalry charge in battle) and the newly-created 43rd Infantry Regiment (which consisted of 1 battalion of troops transferred from the 45th Infantry Regiment). Whether these should be considered as "Asian-American" units depends on your definition. The enlisted men were US nationals, and most were Filipinos rather than Filipino-Americans.
After the Commonwealth of the Philippines was established in 1935 (making the Philippines a self-governing US territory, preparing for independence in 1945), the Philippine Army was set up, growing to 12 divisions by the time the USA and the Philippines entered WWII (2 regular, 10 reserve). These units were not part of the US Army, until July 1941 when the United States Army Forces in the Far East (USAFFE) was created (due to a perceived high chance of war with Japan in the near future). Most units of the Philippine Army became part of the USAFFE, bringing them into the US armed forces command structure.
Shortly after the US entry into WWII (i.e., after Pearl Harbor), Filipino-Americans were allowed to enlist in the US armed forces. Over 1/3 of the Filipino-American population of California applied to enlist; of the 16,000 applications, about 7,000 served. This very high rate of applying to enlist was possible due to the majority of Filipino-Americans being working-age (and military-age) men. From 1943, Filipinos in Hawaii were allowed to enlist (before then, they were classified as workers in an essential industry (i.e., sugar)). Two segregated regiments, the 1st Filipino Regiment and the 2nd Filipino Regiment. The 2nd doesn't appear to have ever grown beyond a single battalion, and was disbanded in early 1944, with its battalion becoming the 2nd Filipino Battalion (Separate). Both the 1st and the 2nd fought in New Guinea in 1944, and went to the Philippines in 1945, mostly conducting mopping-up operations and securing rear areas rather than engaging in front-line combat. Other Filipino-American soldiers served in unsegregated units.
Chinese-Americans mostly served in non-segregated divisions, fighting in Europe and Asia. About 12,000 Chinese-Americans served during WWII, approximately the same proportion of the population as the overall US population. There were two mostly-segregated units in which Chinese-Americans served: the Fourteenth Air Force (established in China in 1943) and the the Chinese-American Composite Wing (CACW). The CACW was officially part of the Chinese Air Force, and most of their aircraft flew with Nationalist markings (and some flew with USAAF markings). The personnel of the CACW were Chinese, Chinese-American and other American. About 25% of Chinese-Americans who served in WWII served as aircrew or ground crew in both of these units. About 5% served in other (unsegregated) USAAF units, and about 70% in unsegregated divisions.
Of the 7,000 Korean-Americans, about 1/3 were non-citizens, and classified as enemy aliens until the end of 1943 (since Korea was under Japanese colonial rule) and couldn't enlist. Compared to other Asian-Americans, relatively few Korean-Americans enlisted, a few hundred of the approximately 4500 citizens. This is about half the service rate of the overall US population (since Korean-Americans were generally enthusiastic supporters of the war against Japan, this was possibly due to many of the citizens being too young to serve). Many of those where served did so as translators, due to their knowledge of Japanese. Their numbers were too few to form useful segregated units, so they served in unsegregated units. (Young-Oak Kim served as an officer in the segregated Japanese-American 442nd Infantry Regiment, leading to, as one might expect, racial tension until his unit went into combat in Italy.) There was one segregated unit, but not in the US Army: about 110 Korean-Americans served in the "Tiger Brigade", in the California State Guard.
References and further reading
1940 census:
Chinese-Americans during WWII:
Korean-Americans during WWII:
Andrew Chae, Between the Devil and the Deep Sea: The Korean American War for Independence (1910-1945), MA thesis, Chapman University, 2020. https://doi.org/10.36837/chapman.000154