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 December 21, 1941, Thailand and Japan signed a military alliance with a secret protocol wherein Tokyo agreed to help Thailand regain territories lost to the British and French. Subsequently, Thailand undertook to 'assist' Japan in its war against the Allies, while at the same time maintaining an active anti-Japanese resistance movement known as the Seri Thai. Approximately 200,000 Asian labourers (mainly romusha) and 60,000 Allied POWs worked on the Thailand–Burma Death Railway.