Circular cablegram M406 LONDON, 30 November 1941, 2.40 a.m.
MOST SECRET AND PERSONAL MOST IMMEDIATE
Following for the Prime Minister.
My telegram M.405. [1]
JAPAN 1. There are important indications that Japan is about to attack Thailand, and that this attack will include a Seaborne expedition to seize strategic points in the Kra Isthmus.
2. The Royal Air Force are reconnoitring on an arc 180 miles from Khota [Bharu] [2] for 3 days commencing 29th November and the Commander in Chief Far East [3] has requested the Commander in Chief of the United States Asiatic Fleet at Manila [4] to undertake Air reconnaissance on the line Manila-Camranh Bay on the same days. The Commander in Chief in the Far East has asked for permission to move into the Kra Isthmus if reconnaissance establishes the fact that escorted Japanese ships are approaching the Isthmus, and he is pressing for an immediate decision on this point. Time is the essence of this plan particularly at this season of the year when the Kra Isthmus is waterlogged.
Consequently great tactical advantage lies with the side which gets there first.
3. Our military advisers fear that operations might lead to a clash which might involve us in war, and they have always emphasized that unless our vital interests were immediately threatened, this should be avoided so long as we have no certainty of United States’ support. In view however of the United States Government’s constitutional difficulties, any prior guarantee of such support is most unlikely.
4. In these circumstances, His Majesty’s Ambassador at Washington [5] has been instructed to explain the position at once to the United States Government and to take the following line. To allow the Japanese to establish themselves so near the Malay frontier would be an obvious threat to Singapore even though at the present season it might not develop at once. We have also to bear in mind the encouragement which the Japanese success would give their extremists. The Japanese appetite would inevitably grow and other Far Eastern peoples would [be] correspondingly depressed. It looks, therefore, as though to ensure the defence of Singapore and for wider reasons we might have to take the proposed action to forestall the Japanese.
5. Lord Halifax is to ask for an urgent expression of the United States Government’s views and has been reminded of the importance of ensuring ourselves of United States support in the event of hostilities.
6. We should be grateful of your views by most immediate telegram.
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1 Dispatched 29 November. On file AA : A816, 19/304/431.
2 Words in square brackets have been corrected/inserted from the London copy on file AA : A2937, Japan-America. 8 March-9 Dec 1941.
3 Air Chief Marshal Sir Robert Brooke-Popham.
4 Admiral Thomas C. Hart.
5 Lord Halifax.
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[AA : A1608, A41/1/5, iv]