Thailand joined the Second World War on the side of the Axis in January 1942, following its invasion by Japan the previous month and the subsequent decision of Thailand's military ruler, Field Marshal Plaek Phibunsongkhram, to ally with rather than resist the Japanese invaders. Puey's studies were as a result interrupted, and he joined the Free Thai Movement resisting the pro-Japanese government, helping to organise the movement in the United Kingdom. Puey was commissioned as a Captain into the British Army and underwent vigorous training with the Special Operations Executive, In November 1944 he parachuted into Chai Nat Province in northern Thailand as part of Operation APPRECIATION, intended to establish contact with the influential and anti-Japanese politician Pridi Banomyong. He was captured almost immediately, and remained technically a prisoner of war until the Japanese surrender in September 1945, though he in fact made contact with Free Thai members of the Thai police and was able to work with them from his jail cell.