In reply to your M.406 [2] decision War Cabinet [3] 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 [4] 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.