46 Sir John Latham, Minister to Japan, to Department of External Affairs
Cablegram 390 TOKYO, 15 August 1941, 4.13 p.m.
1. MY 388.  Your No. 221.  Although swift British-United States reaction following the Japanese entry into South Indo-China has, I believe, disturbed the Japanese Government and may cause them to hesitate to make a move on Thailand, I think that we need to decide now what action should be taken in the event of such a move and should make every effort by diplomatic means to prevent it.
2. As it appears to me four matters require to be taken into account:-
(a) Japanese intentions: If the Minister for Foreign Affairs  can be believed, these (my telegram No. 388) as regards Thailand are at present economic and concern especially supplies of rubber and rice.
(b) United States Policy: I gather from Casey's telegrams following Prime Minister's telegram No. 9, and ending with Casey's No. 40, that United States warning does not apply to an advance into Thailand not continued into other territory. 
(c) Thailand Policy: Latest reports indicating a stiffening and an inclination to resist an attack might be with advantage strongly approved of by a public statement in London.
(d) Our strategical requirements: Can we without imperilling the security of Burma and Malaya, and possibly ourselves, allow the Japanese to establish themselves in Thailand, or in the event of their entering Thailand, must we first forestall them in certain localities.
3. Assuming that our policy is to keep Japan out of the war, I think from (a) we should draw the conclusion (especially since trade from Japan to Germany is no longer possible) that for the time being we should resign ourselves to the Japanese drawing considerable supplies from Thailand and limit our efforts to obtaining what we really need for ourselves.
4. If the answer to (d) is that we must meet this and the Japanese, then in my opinion: (1) we and the United Kingdom should frankly tell the United States so and express the hope that if, in such an event, hostilities resulted they would regard the Singapore life-line as threatened and come to our assistance; (2) United Kingdom should tell Thailand that if they resist, all possible aid will be given, and the twelve aeroplanes they have asked for, at once as a token of security; (3) and perhaps most important, as might prevent Japan taking a step from which they could not withdraw without loss of face, United Kingdom should give some definite but informal warning to Japan of what the result of their entry into Thailand will be but assure them that provided they do not go in we have no intention of doing so.