31 January 1941. As part of the process, secret discussions were held with Thai Prime Minister Plaek Phibunsongkhram, in which the Japanese military sought free passage through Thailand. 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 1940. 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. The situation Phibun faced was that France had now been defeated by Germany, and Britain was heavily engaged in Europe; the United States had until then taken a neutral stance on both the European war and the Japanese war with China; and Japan was a superpower with a growing buildup of forces in French Indochina. 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 1940 also made it difficult for the United States government to support Phibun. Midway through 1941, Phibun sought British and American guarantees of effective support if Japan invaded Thailand. Neither the United Kingdom 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. By August, the United Kingdom and the United States had placed sanctions against Japan. (For further information, see the Hull note and the McCollum memo.) The Japanese sought to have the sanctions lifted by promising not to encroach on Thailand and withdrawing their forces from Indochina, provided the United States withdrew its support for China. This was unacceptable to both Britain and the United States because of its impact on China. Final days In late November, the British became aware of a probable attack on Thailand by Japan because of the rapid buildup of Japanese troops in Indochina. On 1 December 1941, Prime Minister Tojo of Japan stated that he was uncertain where Thailand stood regarding allowing Japanese troops free passage through its territory, but was hopeful a clash could be avoided. Further negotiations took place between the Japanese diplomatic