Believe it or not , the answer is Thailand
"On 8 December 1941,
the Japanese Empire launched an invasion of Thailand, and fierce fighting broke out in Southern Thailand before Phibun (the premier of Thailand) ordered an
armistice and joined the
Axis powers just five hours later. Japan was granted free passage through Thai territory, and on 21 December Thailand and Japan signed a military alliance, with a secret protocol wherein the Japanese government agreed to help Thailand gain further territories throughout southeast Asia. The Thai government then declared war on the United States and the United Kingdom. The United Kingdom, whose colony
Malaya was under immediate threat from Thai forces, responded in kind, but the United States refused to declare war and ignored Thailand's declaration. The
Free Thai Movement was launched both in Thailand and abroad to oppose the Thai government and Japanese occupation. After WW2 ended in 1945, Phibun was ousted and Thailand signed formal agreements to end the state of war with
the Allies"