Prime Minister Plaek Phibunsongkhram followed up border clashes by invading a French Indo-China, under the Vichy regime (after the Nazi occupation of Paris) to regain lost land and settle territorial disputes. The war also bolstered Phibun's program of promoting Thai nationalism. The war ended indecisively, with Thai victories on land and a naval defeat at sea. However, the disputed territories in French Indochina were ceded to Thailand. World War II (1942–1945) To attack British India, British Burma and British Malaya, the Empire of Japan needed to use bases in Thailand. By playing both nations against one another, Prime Minister Phibunsongkhram was able to maintain a degree of neutrality for some time. However, this ended in the early hours of 8 December 1941, when Japan launched a surprise attack on Thailand at nine places along the coastline and from French Indo-China. The greatly outnumbered Thai forces put up resistance, but were soon overwhelmed. By 07:30, Phibun ordered an end to hostilities, though resistance continued for at least another day until all units could be notified. Phibun signed an armistice with Japan that allowed the empire to move its troops through Thai territory. Impressed by Japan's easy conquest of British Malaya, Phibun formally made Thailand part of the Axis by declaring war on the United Kingdom and the United States, though the Regent refused to sign it in the young king's name. (The Thai ambassador to Washington refused to deliver the declaration, and the United States continued to consider Thailand an occupied country.) An active and foreign-assisted underground resistance movement, the Free Thai, was largely successful and helped Thailand to be viewed positively in the eyes of the victorious Allies after the war and be treated as an occupied nation rather than a defeated enemy. Korean War (1950–1953) During the United Nations-mandated conflict in the Korean peninsula, Thailand provided the reinforced 1st Battalion of the 21st Infantry Regiment, Some 65,000 Thais served in Korea during the war. Thai foot soldiers took part in the 1953 Battle of Pork Chop Hill. During the war the battalion was attached at various times to U.S. 187th Airborne Regimental Combat Team and the British 29th Infantry Brigade. The kingdom also provided four naval vessels, the HTMS Bangprakong, Bangpako, Tachin, and Prasae, and an air transport unit to the UN command structure. The Thai contingent was actively engaged and suffered heavy casualties, including 139 dead and more