While Tanaka saw this outcome as resulting from the Emperor's moral leadership, Japanese nationalists used it in terms of freeing Asia from colonizing powers and establishing Japan as the leading influence in Asia. This would, according to imperial propaganda, establish a new international order seeking "co-prosperity" for Asian countries which would share prosperity and peace, free from Western colonialism and domination under the umbrella of a benevolent Japan.
In its final planning stages, the unit was commanded by Colonel Yoshihide Hayashi. In order to do this, they needed to make use of Thai ports, railways, and airfields. They did not want conflict with the Thai military, as this would delay the invasion and significantly reduce the element of surprise.
The Japanese, who wanted to use the Indo-Chinese ports and air-bases, acted as negotiators to bring about a settlement between the French and Thais on 31 January Phibun had responded positively, but his later actions showed he may have been very uncertain, as he had concluded the British—Thai Non-Aggression Pact on 12 June By February, the British were beginning to suspect the Japanese were planning to attack their possessions in Southeast Asia and were concerned Japan might set up bases in Thailand to that end.
Phibun could have decided he had little choice, as his own forces would have been unable to defeat the Japanese by themselves. Thailand's invasion of French Indochina in also made it difficult for the United States government to support Phibun. Neither Britain nor the United States could give them, although British Prime Minister Winston Churchill was in favour of a public warning to Japan that an invasion of the Southeast Asian kingdom would result in a British declaration of war.
However, the United States was unwilling to agree to this, and Britain was not prepared to make it alone. For further information, see the Hull note and the McCollum memo.
