They existed, but were not prominent because Japan's recovery was swift. Those who did fight alongside other Asian parties from 1945 to 1970 generally did so because of Pan-Asianist ideology or fear of persecution for war crimes, not (solely) for money.

Economic prospects in Japan in the late 1940s were generally not as bad as often imagined. The response of the government to the post-war crises, namely inflation, a shortage of electric power, and a housing shortage were decisive. The Finance Ministry quickly began an inflationary policy of overloading to the 12 major local banks of Japan to inject capital back into the economy. These loans were coordinated by the Ministry of Commerce and Industry (after 1949, the Ministry of International Trade and Industry) into coal and electric power sectors in a strategy known as "priority production". For a short while, Finance Minister Ishibashi Tanzan also became a "development loan" strategy where coal mines and electric power companies got loans directly out of the public budget, but this was cancelled by the Americans who believed in budget austerity and Ishibashi was later purged when one of his political opponents falsely convinced the Americans that he was a war criminal (he was actually an anti-imperialist and was nothing more than a newspaper editor). "Priority production" was an enormously successful strategy and Japan regained pre-war levels of production by 1951.

However, until Japan regained its independence in 1952, many military officers were afraid of persecution for war crimes and found service in other Asian armies. The most famous Japanese mercenary of the Cold War, Tsuji Masanobu, started his career for this reason. The staff officer who engineered the victory in Malaya, Tsuji was involved in numerous war crimes and knew he would be jailed or shot if he came home. Instead, he found his way (ironically) into the service of Chiang Kai Shek, to whom he was a military adviser. Tsuji returned to Japan after the Americans had left, and quickly secured for himself a seat in the Diet. However, in the 1960s he fell out of favor because of leaked revelations about his wartime record. Leaving for Indochina, Tsuji was never seen again. The CIA believed that the life-long Pan-Asianist had found himself a new job as a military advisor for the North Vietnamese.

Tsuji was far from alone, and, in the late 1940s, thousands of Japanese like him would serve with both the Nationalists and the Communists during the Chinese Civil War. Former Japanese radio operator Hattori Yasuzo wrote one of the most detailed accounts of the phenomenon. After WW2, American officials were keen to have the Japanese surrender to the Nationalist Chinese instead of the Americans, that way their weapons would fall into American hands. Hattori recounted that the Kuomintang treated Japanese prisoners with unusual kindness, going as far as to make them feel as if they were not prisoners so as to not dishonor them. It soon became clear why. The KMT offered Hattori's entire unit paid positions in its own hierarchy. 500 of the Japanese accepted, and were made officers in the Chinese army. Hattori noted that many accepted not only for monetary reasons, but because they feared a life outside of war having lived nothing but war for years, and because Japanese culture had strict rules in regards to repaying goodwill - something the KMT seemed to be keenly aware of.

The entire operation was kept secret from the Americans to avoid bad press. Hattori became "Lt. Pai Syu-gen". His unit was originally tasked with restarting war production in the Baotou region and managing KMT logistics, but they were transitioned to a combat role as soon as the Communists advanced. The Japanese were put in command of a force of 3,000 Chinese draftees. Indoctrinated in a military ethos that did not permit retreat, the Japanese refused to evacuate Baotou and were quickly surrounded. During the battles for the city, Hattori noted that there were Japanese fighting for the Communists as well. The Japanese on both sides quickly came to an agreement not to shoot each other, identifying themselves through slight modifications to their uniforms.

Sometimes, the Nationalists' employ of Japanese mercenaries did not emerge from these kinds of consensual agreements, but rather through deception. After becoming a Kuomintang POW in August 1945, Captain Sasaki Harutaka and his entire unit were ordered to march North of Nanjing, still in their uniforms and not having been disarmed. When they arrived at what they thought would be their POW camp, they were instead greeted by a Japanese-speaking KMT official, who ordered them to attack the Communists. Orders had also come down from KMT headquarters ordering the "POWs" to defend the position until relief arrived. It never did. Sasaki and his unit would not return to Japan until almost a year after the country's surrender.

Elsewhere in Asia, Japanese forces often held out against "the enemy", however ambiguous that definition may have been, on their own when they needed to and in collusion with local forces where possible. In Vietnam, up to 5,000 Japanese stayed behind and helped to train the Viet Minh, which up to that point had limited combat and military expertise. These included Major Ishii Takuo, who deserted from his post in the Japanese 55th Division in Burma following Japan's surrender, made his way to Vietnam, and was made a colonel in the Viet Minh. In 1947, Colonel Ishii engineered an ambush that killed 70 French men, where the same year French forces in Hue reported battling an assault from 150 Japanese.

While many Japanese were scattered as officers, logisticians, and medics throughout Viet Minh forces, there was one large concentration. 1,500 Japanese veterans in 1945 were organized by Vietnamese General Vo Nguyen Giap into a coherent unit commanded by Colonel Mukayama of the Imperial General Staff. As in China, the movations of Japanese forces were a mix of financial concerns, pan-Asianism, warrior identity, and fear of war crimes tribunals. At least 47 members of the Kempetai, Japan's feared military police, served with Giap in the late 1940s. Meanwhile, Mukayama was described by the French as "fanatically anti-white" and likely served for ideological reasons.

However, as in China, some Japanese 'mercenaries' served only because of coercion or deception. This was especially true of logistical experts and medical staff who could be easily supervised. After 1951, the Viet Minh started to repatriate these individuals.

The situation in Indonesia mirrored that in Vietnam. There, the Japanese-backed wartime government led by Sukarno survived the war and won its war of independence against the Dutch. Around 320 Japanese veterans fought for Sukarno after the war, led by Ichiki "Abdul Rahman" Tatsuo. This relatively small number compared to Vietnam and China is due to the fact that comparatively few Japanese were deployed in the main islands of Indonesia, and the territory was geographically distant and harder for deserters to get to. Historians are divided in regards to their motivations, but once again the true answer seems to be a mix. Some were worried about a lack of career prospects in Japan, others were indoctrinated into seeing themselves as warriors, others were Pan-Asianists, and others feared war crimes tribunals.

Sources:

Goscha, Christopher. Belated Asian Allies.

Johnson, Chalmers. MITI and the Japanese Miracle.

Hayashi, Eiichi. Zanryuu Nihon-hei no Shinjitsu.

Satoshi, Nakano. Japan's Colonial Moment in Southeast Asia 1942–1945: The Occupiers' Experience.

Strauss, Ulrich. The Anguish of Surrender: Japanese POWs of World War II.

Devillers, Philippe. History of Vietnam from 1940 to 1952.