In World War Two did Japan field any military units made up of conquered peoples, such as Koreans, Chinese or Philippinos?

To an extent, as much as you could consider Okinawa a "conquered" territory. The Ryukyu Kingdom had been made a vassal state of the Japanese Empire in the 1870s before being annexed and made a prefecture. Many Okinawans, particularly those of indigenous descent, considered themselves "Okinawan" first and "Japanese" second, or not "Japanese" at all. This led to ill feelings from those who lived on the home islands or came to Okinawa. Some considered the Okinawans as second-class citizens and not truly "Japanese." The Ryukyuan and mainland Japanese languages are in the same language family (Japonic), but are not mutually intelligible with each other; interestingly enough, even the various Ryukyuan languages are not mutually intelligible among themselves! r/AskHistorians' resident Okinawan history expert appears to not have posted in a while, but there may be other users versed in Japanese history who can fill in more about the relations between ethnic Japanese and Okinawans during this time period.

During the Battle of Okinawa, the Imperial Japanese Army and Navy units on the island conscripted about 39,000 native Okinawans into their ranks. In July 1944, a voluntary village-based militia, known as the Boeitai was formed, which was to be incorporated into units of the Imperial Japanese forces on the island if the Americans decided to invade it. The number of Boeitai issued Japanese military equipment and uniforms is estimated at between 17,000 and 20,000. About 15,000 other civilians were used as non-uniformed laborers. 1,500 to 1,700 middle school and high school-aged boys, aged 14 and older, were organized into Tekketsu Kinnotai ("Blood and Iron for the Emperor") volunteer units. The Boeitai, laborers, and boys were often treated poorly by their ethnic Japanese commanders, who asserted to them that their families would be killed if they tried to surrender. Since the militia was not imbued as harshly with the "no surrender" mantra as compared to their regular military counterparts, many of the conscripted native Okinawans deserted and attempted to blend back into the local population, and were more willing to surrender rather than fight to the death when confronted by American troops. The cost on Okinawa for the Japanese was horrific. About 100-110,000 Japanese combatants, including 15,000 of the conscripted Boeitai were killed. Only 7,400 military personnel and about 3,400 Boeitai and unarmed laborers were taken prisoner. Anywhere from one-tenth to one-third of the island's pre-war civilian population of about 300,000 died or went missing, either by American or enemy action, or by their own hand after being urged to commit suicide by the occupying Japanese out of fear what the Americans would do to them if captured. The overall casualty count was confused by the fact that many of the dead were conscripted Okinawans wearing Imperial Japanese military uniforms. The U.S. military accepts a minimum number of about 42,000 civilian dead (counted bodies) from all causes, but other estimates for the number of civilian dead and missing are as high as 100,000.

Enemy losses for the entire Okinawa campaign were placed at 107,539 counted dead and an estimated 23,764 more which were assumed to have been sealed in caves or buried by the Japanese themselves. In addition, a total of 10,755 of the enemy had been captured; some of this number had surrendered. As the overall Japanese casualty total of 142,058 was "far above a reasonable estimate of military strength on the island," Tenth Army intelligence agencies presumed that approximately 42,000 of these casualties were civilians that had been unfortunately killed or wounded in American artillery, naval gunfire, and air attacks on enemy troops and installations while the natives had been in the proximity.

Sources:

Appleman, Roy E., James M. Burns, Russell A. Gugeler, and John Stevens. U.S. Army in World War II, The War in the Pacific, Okinawa: The Last Battle. Washington, D.C.: United States Army Center of Military History, 1948.

Frank, Benis M., and Henry I. Shaw. Victory and Occupation: History of U.S. Marine Corps Operations in World War II, Volume V. Washington, D.C.: Historical Branch, G-3 Division, Headquarters U.S. Marine Corps, 1968.

Huber, Thomas M. Japan's Battle of Okinawa, April-June 1945. Fort Leavenworth: Combat Studies Institute, United States Army Command and General Staff College, 1990.

Morrow, James D. Order within Anarchy: The Laws of War as an International Institution. Cambridge: Cambridge University Press, 2014.

Besides Okinawans, as mentioned by u/the_howling_cow, a significant number of Koreans and Taiwanese also served in the Japanese military. To give some background, Taiwan was annexed by Japan in 1895, followed by Korea in 1910. In both colonies, Japanese authorities implemented Japanisation policies, such as mandatory education of the Japanese language, encouragement of Japanese dress codes, and the promotion of Shintoism. This resulted in complicated ethnic identities for the youth who had only known Japanese rule, especially in Taiwan where there was an equally strong resistance towards a Han Chinese identity. Therefore, at the start of the Sino-Japanese conflict in 1937, there were portions of the Korean and Taiwanese population who volunteered for military service out of a sense of patriotism, while others were coerced.

However, the Japanese military never fully trusted (or believed it was even necessary - after all, many in Tokyo held the belief the Chinese Nationalist government would collapse within a year) personnel recruited from its colonies. Initially, Koreans and Taiwanese volunteers were relegated to non-frontline roles. Besides providing logistics support, they were also involved in the construction of airfields and translation duties. A group of Taiwanese volunteers even served as an agricultural unit in China to grow crops for Japanese frontline units. Many Koreans and Taiwanese also served as POW camp guards in Burma, Taiwan, Philippines and mainland Japan. Some of them committed extreme acts of brutality - 129 Koreans and 173 Taiwanese were convicted of Class B and C war crimes at the International Military Tribunal for the Far East, resulting in the execution of 40 combined. Despite the inherent discrimination of the Japanese military system, there were several Koreans and Taiwanese who did rise to high command. Crown Prince Yi Un, the son of the last Korean Joseon Emperor, commanded the Japanese 51st Division, and was later promoted to Lieutenant General of the Army Air Force, commanding the 1st Air Army.

While there were no frontline units comprised of Koreans or Taiwanese before 1942, there were some paramilitary units serving in occupied regions. The Gando Special Force was a group of ethnic Koreans trained in anti-insurgency warfare by the Kwantung Army. Serving in the Jiandao region of Manchuria from 1938 onwards against mixed Chinese and Korean pro-communist guerrillas (notably the founder of North Korea, Kim Il-Sung), they gained a particularly unsavoury reputation for brutality. The Japanese Special Naval Landing Forces based on Hainan Island in South China had several mixed battalions comprised one third of forty year old Japanese reserves, and two thirds twenty year old Taiwanese volunteers. These battalions were engaged in a drawn out mini-war on the island with Communist guerrillas which lasted until the end of the Second World War.

The Takasago Volunteers, formed from members of the Taiwanese aboriginal tribes, participated in the New Guinea and Philippines campaigns. Originally, they were recruited to provide logistics support. However, their expertise in traversing forested areas and reconnaissance soon earned them the respect of their Japanese superiors, and the volunteers were reformed into special forces units. With their new status came new privileges, such as a higher salary than even Japanese frontline troops. Many of these units participated in the kamikaze attacks at the Battle of Lyete Gulf, crash-landing Japanese bombers on American airfields to perform commando raids. Their high-risk missions resulted in a disproportionate amount of casualties - out of the 7,000 volunteers, 3,000 never made it back to Taiwan.

With the start of the Pacific War, Japan started to recruit volunteers in Taiwan and Korea to serve as frontline soldiers in early 1942. Under the Army Special Volunteers System, volunteers underwent six months of training before joining Japanese divisions as replacements. This programme was highly competitive: in Korea, only 4,000 men were chosen out of 250,000; in Taiwan, 1,000 out of 425,000 in 1942. As mentioned before, there was a genuine sense of patriotism that induced young men to volunteer. A desire to no longer be second-class citizens, attractive pay and better food rations were alternative reasons to volunteer. At the same time, there was a coercive element involved in recruitment. In Korea, Japanese records show only 35% volunteered of their own volition (one can equally argue 35% is a surprisingly high number for a colony). In Taiwan, there was also strong anger against American bombing campaigns - at the height of the bombing campaign in 1944, a high of 750,000 Taiwanese tried to enlist. Additionally, the Japanese Navy was also eager to recruit Taiwanese sailors, with the total number of enlisted sailors at 11,000 actually double those of army recruits at 5,000. Eventually in 1945, a conscription system was set in place with Taiwan and Korea isolated by Allied advances. Both men and women received military training, and there was a strong expectation that the conscripted would commit gyokusai (literally shattered jewels) suicide attacks against any amphibious landing. Thankfully, or horrifyingly, the detonation of atomic bombs on Hiroshima and Nagasaki brought the war to an end before any of these operations could be carried out.

In Korea and Taiwan, these combatants were viewed as the ‘lost generation’. The South Korean and Guomindang governments viewed returning soldiers as unreliable elements at best, traitors at worst, while the Japanese government explicitly rejected any relations with its former soldiers. Thus, they existed in some sort of limbo, rejected by their home country and their former occupiers. However, with the discovery of Teruo Nakamura in 1974, the last known Japanese hold-out to surrender, the media gained an interest in the stories of Taiwanese and Korean soldiers. Teruo Nakamura was originally part of the Taiwanese Amis aboriginal tribe, and served as a Takasago Volunteer in Indonesia. His rediscovery became a media sensation, and directly brought public attention upon this group of forgotten soldiers. In both Taiwan and Korea, veteran associations were formed, and began to push for Japanese compensation. This was achieved for Taiwanese soldiers in 1987, and for Koreans in 2002. However, the compensation received was much less than those for ethnically Japanese soldiers.

Controversy surrounding veterans continue to this day. One particular issue is the enshrinement of Korean and Taiwanese servicemen at the Yasukuni shrine. Veteran associations have repeatedly called for the return of urns to their respective homelands - these requests have been repeatedly ignored. Discrimination against veterans in their home countries still remain. In 2006, conservatives in Taiwan pushed for the destruction of the Takasago Volunteers Memorial Park - this was only stopped by joint efforts from indigenous associations and academics, who countered conservative rhetoric by advocating the ‘Freedom to Remember.’ Khoo Chau Ing committed self-immolation in front of the Taiwanese Tomb of the Unknown Soldier in 2008 to protest against neglect of former Taiwanese Imperial Army servicemen, and specifically, against the lack of commemoration sites. His actions prompted the creation of the Kaohsiung War and Peace Memorial Park, which commemorates Taiwanese soldiers involved in the Second World War and the Chinese Civil War, as well as comfort women and conscripted military labourers. Sadly, the service of Taiwanese and Korean soldiers in Japan’s wars continues to be a source of sorrow and grief.

​

Sources:

Zhang Zi-jing. Zaijian Hainan dao: taiji Riben bing Zhang zijing taipingyang zhongzhan huiyi lu [Goodbye Hainan Island: Taiwanese Imperial Japan Serviceman Zhang Zi-jing’s Remembrances of the Pacific War]. Taipei: Yuanzhu weihua, 2017.

Chen Bai-zong. “Rizhi shiqi taiji riben bing renwu ji qi jingyu zhi tantao ― yi fenpai zhi zhongguo, nanyang zhanqu weili [An Exploration of the Missions and Experiences of Taiwanese Imperial Japan Servicemen: Using Deployments in China and the South Pacific as Examples].” Xinbei da shixue 4 (2006), 33-53

Chen Chih-Fan. “Zhanzheng yu jiti baoli - Gaosha yiyongdui xingxiang de wenxue zaixian yu jiangou [The War and Collective Violence: The Reproduction and Construction of the Image of the Tasaka Pioneer Force in Literature].” Taiwan wenxue yanjiu xuebao 26 (2018), 157-184

Fujitani, Takashi. Race for Empire: Koreans as Japanese and Japanese as Americans during World War II. ACLS Fellows' Publications. Berkeley, Calif.: University of California Press, 2011.

Palmer, Brandon. Fighting for the Enemy : Koreans in Japan's War, 1937-1945. Korean Studies of the Henry M. Jackson School of International Studies. 2013.

Ching, Leo T. S. Becoming "Japanese" : Colonial Taiwan and the Politics of Identity Formation. Berkeley: University of California Press, 2001.