Circular cablegram M424 LONDON, 5 December 1941, 4.21 p.m.
IMMEDIATE MOST SECRET
Following for the Prime Minister.
My Circular M.421 of 5th December [1] and M.422. [2] The position is that we have now received an assurance of armed support from the United States.
(a) If we find it necessary either to forestall a Japanese landing in Kra Isthmus or to occupy part of the Isthmus as a counter to Japanese violation of any other part of Thailand;
(b) If the Japanese attack the Netherlands East Indies and we go at once to support the latter;
(c) If the Japanese attack us.
2. We have accordingly instructed the Commander in Chief, Far East [3], that he should take action as he has suggested (see paragraph 2 of my Circular M-406 of 30th November [4]) without reference to us if either (a) He has good information that a Japanese expedition is advancing with the apparent intention of landing on Kra Isthmus, or (b) The Japanese violate any other part of Thailand.
3. The Commander in Chief, Far East, has also been authorised in the event of a Japanese attack on the Netherlands East Indies to put into operation without further reference to us the plans already agreed with the Netherlands East Indies.
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1 In fact dispatched on 4 December. On file AA:A981, Japan 178.
2 Dispatched 5 December. On file AA:A981,Japan 185B, iii.
3 Air Chief Marshal Sir Robert Brooke-Popham.
4 Document 139.
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[AA:A981, JAPAN 185B, iii]