Thailand waged the Franco-Thai War in October 1940 to May 1941 to reclaim territory from French Indochina. Japanese forces invaded Thailand an hour and a half before the attack on Pearl Harbor (because of the International Dateline, the local time was on the morning of 8 December 1941). Only hours after the invasion, Prime Minister Field Marshal Phibunsongkhram ordered the cessation of resistance against the Japanese. An outline plan of Japan-Thailand joint military operations, whereby Thai forces would invade Burma to defend the right flank of Japanese forces, was agreed on 14 December 1941. On 21 December 1941, a military alliance with Japan was signed and on 25 January 1942, Sang Phathanothai read over the radio Thailand's formal declaration of war on the United Kingdom and the United States. The Thai ambassador to the United States, Mom Rajawongse Seni Pramoj, did not deliver his copy of the declaration of war. Therefore, although the British reciprocated by declaring war on Thailand and considered it a hostile country, the United States did not.