On 8 December 1941, Imperial Japan launched attacks on Southeast Asia and the Allied possessions in the region, opening the Pacific War.
This included amphibious landings in Thailand and an invasion across the border from French Indochina.
After initially resisting, the Thai government reluctantly agreed to let the Japanese pass through the country and use its military bases to strike other Allied possessions in the region, culminating in the Battle of Malaya.

When Field Marshal Plaek Phibunsongkhram issued a declaration of war against Britain and the United States in January 1942, Pridi refused to sign it, as he served as the Regent for the young monarch, who was studying in Switzerland.
In this capacity, Pridi built the anti-Japanese underground, the Free Thai Movement ("Seri Thai") network, in Thailand.
Code named "Ruth", he established contact with the Allies and Thai resistance organisations in Britain and the United States.
As the war progressed and the fortunes of the Japanese declined, public dissatisfaction grew and Phibun was forced to resign as prime minister in 1944.

Khuang Abhaiwongse, a liberal lawyer and member of Seri Thai, was chosen to be prime minister due to "his ability to dissemble with the Japanese" to shield the growing Seri Thai movement while at the same time improving superficial relations with the Japanese occupiers.

When Japan's surrender ended the war, the Seri Thai-dominated government immediately acted to "restore the pre-war status quo".
As regent, Pridi termed "the declaration of war illegal and null, and void" as improperly made, and repudiated all agreements made with Japan by Phibun.

When Lord Louis Mountbatten, the Supreme Commander, Southeast Asia, visited Bangkok in late-1945, he recorded a tribute to Pridi in which he said that there had existed a unique situation wherein "the Supreme Allied Commander was exchanging vital military plans with the Head of a State technically at war with us".
On 8 December 1941, Imperial Japan launched attacks on Southeast Asia and the Allied possessions in the region, opening the Pacific War. This included amphibious landings in Thailand and an invasion across the border from French Indochina. After initially resisting, the Thai government reluctantly agreed to let the Japanese pass through the country and use its military bases to strike other Allied possessions in the region, culminating in the Battle of Malaya.
When Field Marshal Plaek Phibunsongkhram issued a declaration of war against Britain and the United States in January 1942, Pridi refused to sign it, as he served as the Regent for the young monarch, who was studying in Switzerland. In this capacity, Pridi built the anti-Japanese underground, the Free Thai Movement ("Seri Thai") network, in Thailand. Code named "Ruth", he established contact with the Allies and Thai resistance organisations in Britain and the United States. As the war progressed and the fortunes of the Japanese declined, public dissatisfaction grew and Phibun was forced to resign as prime minister in 1944.
Khuang Abhaiwongse, a liberal lawyer and member of Seri Thai, was chosen to be prime minister due to "his ability to dissemble with the Japanese" to shield the growing Seri Thai movement while at the same time improving superficial relations with the Japanese occupiers.
When Japan's surrender ended the war, the Seri Thai-dominated government immediately acted to "restore the pre-war status quo". As regent, Pridi termed "the declaration of war illegal and null, and void" as improperly made, and repudiated all agreements made with Japan by Phibun.
When Lord Louis Mountbatten, the Supreme Commander, Southeast Asia, visited Bangkok in late-1945, he recorded a tribute to Pridi in which he said that there had existed a unique situation wherein "the Supreme Allied Commander was exchanging vital military plans with the Head of a State technically at war with us".
Pridi Banomyong