On December 8, 1941, less than two hours after the attack on Pearl Harbor, Japanese forces began invading Thailand. Despite Thailand's strong national pride, the government had no recourse but to surrender to Japanese demands and allow their troops to occupy the country. The military agreement between Thailand and Japan spontaneously mobilized an underground resistance movement that gained in strength and momentum as the war progressed. Led by Pridi Banomyong, the resistance grew into a mass movement throughout the country, including many prominent figures in the government, the military, and the police. Organized resistance efforts sprang up among Thai students in the U.S. and England as well, training them for guerrilla operations and intelligence missions inside Thailand. The Seri Thai, or Free Thai Movement, became a major political force once the war was over, effectively blocking the attempt by some allied countries to extract concessions and take reprisals against Thailand.