157 Lord Cranborne, U.K. Secretary of State for Dominion Affairs, to Mr John Curtin, Prime Minister
Circular cablegram M418 LONDON, 3 December 1941, 9.57 p.m.
MOST IMMEDIATE MOST SECRET
Following for the Prime Minister.
My circular M.412. 
JAPAN After consideration of Lord Halifax's account of his discussion with the President the following reply is being despatched to Lord Halifax this evening. Text begins.
1. We entirely agree with President Roosevelt that we and the United States Government (and the Netherlands Government) should be clear as to what action we shall respectively take in the various situations likely to arise.
2. Of the hypotheses in paragraph 6 of your telegram the first two seem to us for practical purposes indistinguishable.  All of our information goes to show that reinforcements have been arriving for some time and are still continuing. The only question therefore appears to be what we should do if the Japanese reply to President Roosevelt's enquiry is unsatisfactory. We feel that the proper reaction would be simultaneous warning to Japan by the United States, the Dutch and ourselves, to the effect that if she uses Indo Chinese territory as a base for further aggression she will do so at her peril. If such warnings are given it is necessary to be clear as to the action which we shall take if they are ignored. We read the President's assurance of support recorded in paragraph 8 of your telegram  to mean armed support and on this assumption we should ourselves be prepared to put into effect the operation referred to in my circular M.4064 if there were a direct Japanese attack or threat of immediate attack on Kra Isthmus. Under hypothesis (c) the threat to Singapore might be less immediate, but we should still think it wise to put into operation the same plan provided that we had a similar assurance that we would have the armed support of the United States if our action resulted in hostilities with Japan. If the President wishes to suggest any other form of action in which he would be disposed to participate, we should of course be glad to consider it. We note particularly the President's statement that in any direct attack on ourselves or the Dutch we should all be together. We fully endorse this statement.
3. We have already been considering the possibility of some arrangement with the Thai Government under which our entry into Kra Isthmus at whatever stage it might take place would be by their invitation. The difficulty at present is that we are not militarily in a position to give direct assistance to the Thai Government in the protection of the rest of their territory. The proposal to occupy a very small part of Kra Isthmus is therefore unlikely to appeal to them and we fear that the same applies to an undertaking from us to guarantee their ultimate full sovereignty and independence. It would, we feel, be asking a good deal of them to expect them to accept virtual certainty of partial extinction in order to ensure their ultimate independence.
4. The Thai Prime Minister  has forcibly represented to us that the only way to save Thailand is by a public warning to Japan by the United States and ourselves that if she went to war with Thailand she would find herself at war with us both as well. If he knew that a warning of the type suggested in paragraph 2 above had been delivered even though it were not made publicly his outlook might change. We feel however that any communication to him about our plans in Kra Isthmus would be useless until such a warning has been delivered and there is in addition very great risk of leakage to the Japanese. For the moment we are confining our actions to informing Sir J. Crosby  of our plan so that he may be able to make a communication at short notice.
5. We would propose to make it clear in any such communication or in any announcement which might accompany our operation that we should restore in full to Thailand any territory which we might occupy and that our aim is to ensure full independence, sovereignty and territorial integrity of Thailand. Text ends.