On 12 June 1940, in Bangkok, the United Kingdom and Thailand sign the Non-Aggression Pact — the five-year bilateral instrument amid Bangkok’s alignment with Japan and British caution in East Asia.

Ratifications were exchanged on 31 August 1940; the text was registered in the League of Nations Treaty Series in 1941. Articles forbid mutual war, annul assistance obligations against the other party and protect territorial control.

Context: after Japan’s invasion of China, the Craigie–Arita formula and the Burma Road closure, London had avoided open confrontation with Tokyo. During the Battle of France, the pact with Thailand sought not to antagonise Japan.

Despite the pact, Thailand declared war on the United Kingdom on 25 January 1942.

For Southeast Asia, 12 June 1940 marks British temporising diplomacy before the Pacific War.

The text appears in LNTS vol. 203.

The Anglo-Thai Pact of 12 June 1940 remains the non-aggression agreement broken by the 1942 declaration of war.

Image: Bangkok; Wikimedia Commons. Cropped to 16:9.

Source consulted: Anglo-Thai Non-Aggression Pact. Treaty text: League of Nations Treaty Series — Anglo-Thai Non-Aggression Pact.