The Axis-aligned states which assisted Japan included the authoritarian government of Thailand, which formed a cautious alliance with the Japanese in 1941, when Japanese forces issued the government with an ultimatum following the Japanese invasion of Thailand. The leader of Thailand, Plaek Phibunsongkhram, became greatly enthusiastic about the alliance after decisive Japanese victories in the Malayan campaign and in 1942 sent the Phayap Army to assist the invasion of Burma, where former Thai territory that had been annexed by Britain were reoccupied (Occupied Malayan regions were similarly reintegrated into Thailand in 1943). The Allies supported and organized an underground anti-Japanese resistance group, known as the Free Thai Movement, after the Thai ambassador to the United States had refused to hand over the declaration of war. Because of this, after the surrender in 1945, the stance of the United States was that Thailand should be treated as a puppet of Japan and be considered an occupied nation rather than as an ally. This was done in contrast to the British stance towards Thailand, who had faced them in combat as they invaded British territory, and the United States had to block British efforts to impose a punitive peace.