163 Lord Cranborne, U.K. Secretary of State for Dominion Affairs, to Mr John Curtin, Prime Minister
Circular cablegram M426 LONDON, 5 December 1941, 8.20 p.m.
IMPORTANT MOST SECRET
Following for the Prime Minister:-
THAILAND My Circulars M.418  paragraphs 3-5, M.421 paragraph 7 , and M.422  [paragraph 5]. 
1. We have explained to His Majesty's Minister at Bangkok  for his most secret and personal information the nature of the Kra Isthmus plan and the circumstances in which it would be put into operation. We are also consulting him on the following lines:-
(1) It is important that if and when the operation is carried out it should not meet with Thai resistance. It is in our view our best first strategic move in circumstances envisaged, not excluding further possibilities, and represents the best means of helping Thailand.
(2) We have it in mind therefore in order to prepare the way to give Thailand an assurance that in the event of a Japanese attack we will help them to the best of our ability. 
(3) We hope that this promise of assistance might encourage Thailand to resist Japan and that the Thailand Government if compelled to abandon Bangkok would transfer themselves to British territory while their troops retired southwards to link up with our own.
(4) We understand the Thai position to be that no military arrangement of any kind is practicable without a definite warning to Japan by the United States and ourselves.  We are consulting the United States Government about the possibility of a warning, but Sir J. Crosby should make no approach to the Thai Government pending further instructions.
2. A further telegram will be sent on the receipt of Sir J.
Crosby's reply. In the meantime we have informed His Majesty's Ambassador at Washington  that we are consulting Sir J. Crosby in this sense. We hope (see paragraph 5 of Circular M.422) that the United States Government may feel able to join with us in the assurance as in paragraph 1 (2) above.