20th century
The losses initially included Penang and eventually culminated in the loss of four predominantly ethnic-Malay southern provinces, which later became Malaysia's four northern states, under the Anglo-Siamese Treaty of 1909.
In 1932, a bloodless revolution carried out by the Khana Ratsadon group of military and civilian officials resulted in a transition of power, when King Prajadhipok was forced to grant the people of Siam their first constitution, thereby ending centuries of absolute monarchy.
During World War II, the Empire of Japan demanded the right to move troops across Thailand to the Malayan frontier. Japan invaded the country and engaged the Thai Army for six to eight hours before Plaek Pibulsonggram ordered an armistice. Shortly thereafter Japan was granted free passage, and on 21 December 1941,