Treaty between Thailand and Japan (1940)

The Treaty between Thailand and Japan Concerning the Continuance of Friendly Relations and the Mutual Respect of Each Other's Territorial Integrity was concluded in Tokyo on June 12, 1940 between the Thai and the Japanese governments. It was a step in the unfolding cooperation between the Thai and the Japanese governments, which eventually made them allies during the Second World War.

Ratifications were exchanged in Bangkok on December 23, 1940, and it became effective on the same day. The treaty was registered in League of Nations Treaty Series on July 26, 1941.

Background
Following the Japanese invasion of China in 1937, the Japanese government began cultivating relations with the Thai government. This trend was intensified during the battle of France (May–June 1940), as the German forces advanced into France, a situation the Japanese government was keen to exploit in order to gain control over French Indochina. The Kingdom of Thailand had a common border with Indochina, and for that purpose was viewed by Tokyo as a possible ally against the French control of that area.

Terms of the treaty
Article 1 provided for mutual respect of each other's sovereignty by the Japanese ant the Thai governments. Article 2 provided for joint consultation between the Japanese and the Thai governments on matters of common concern. Article 3 stipulated that in case any of the parties is under attack by a third party, the other signatory party shall not lend any assistance to the attacking third party. Article 4 provided for ratification of the treaty. Article 5 stipulated the treaty shall remain in force for five years, with the possibility to extend it further.

Aftermath
The growing cooperation between the Thai and the Japanese governments in the wake of the treaty led the Thai government to wage a brief and successful war against French Indochina in 1940-1941, which resulted in some territorial conquests for Thailand, made with Japanese assistance. In general, this trend facilitated the gradual process of Japanese take over of Indochina throughout 1941, which was one of the main causes for Japanese entry into the Second World War. The Thai government, in return, was compelled to join the Japanese government in a treaty of alliance in December 1941.