In reply to your M.406  decision War Cabinet  is as follows:(I) It is plain that the intention of Japan in attacking Thailand will be to establish bases from which to attack China, Burma and Malaya.
(2) We agree that defence of Malaya justifies giving an authority to the Commander-in-Chief, Far East  to move troops into the Kra Isthmus, providing reconnaissance definitely establishes that Japanese expeditionary force is approaching Isthmus.
(3) It is assumed that the British forces to be moved into Isthmus will be adequate to ensure the defence of this region without prejudicing the effective defence of Malaya itself (4) All endeavours should be made to obtain the prior consent of Thailand to the course proposed, but if such consent cannot be obtained, the defence of Malaya justifies action without such consent.
(5) United States armed co-operation is so desirable that she should be advised of our intentions. We realise the constitutional difficulties of securing prior undertakings, but U.S. should at least be asked whether any objections are raised to our proposals.
(6) In view of most valuable initiative and assistance from United States in talks with Japan, we think that the question of prior warning to Japan or other similar diplomatic action should rest with the United States.
(7) We again repeat our opinion that even at this late stage a further endeavour should be made to encourage United States to establish a modus vivendi with Japan, which can be made satisfactory to China as well as to other powers concerned.