In the wake of their rapid advance into Southeast Asia after 8 December 1941, the Japanese began to implement a new monetary policy in the region to replace the long-established Western financial order that was being swept away. The purpose of this article is to discuss aspects of this monetary policy, in particular how it was carried out in regards to Thailand during the first year of the war, the period during which Thai–Japan wartime relations became established. In the wake of their rapid advance into Southeast Asia after 8 December 1941, the Japanese began to implement a new monetary policy in the region to replace the long-established Western financial order that was being swept away. The purpose of this article is to discuss aspects of this monetary policy, in particular how it was carried out in regards to Thailand during the first year of the war, the period during which Thai-Japan wartime relations became established. With the outbreak of war the most urgent need of the Japanese in their relations with Thailand was to convert that country into a suitable rear area for their campaign in Malaya and for the one upcoming against Burma. To this end the Japanese and Thai governments signedon 8 December an agreement permitting Japanese forces to pass through Thai territory to attack Malaya; but equally important the agreement also stipulated that Thailand would afford the Japanese forces all necessary convenience for their passage through Thailand. This stipulation was the key that opened the way for the Japanese to pursue their plans towards Thailand. By passage, of course, the Japanese did not simply mean the travel of their army through Thai territory to attack Malaya and later Burma (although superficially this is what was implied).