347 Lord Cranborne, U.K. Secretary of State for Dominion Affairs, to Sir Geoffrey Whiskard, U.K. High Commissioner in Australia
Cablegram Z66 LONDON, 11 March 1941,9 p.m.
MOST SECRET AND PERSONAL
Please give the following message to the Prime Minister  for his most secret and personal information. BEGINS.
1. We have had in mind for some time that it would be desirable to persuade the United States Government that one of the most effective methods of checking Japanese southward movement would be a joint declaration by the United States and ourselves that an attack on the Netherlands East Indies or on British possessions in the Far East would be regarded as a casus belli by both the United States and ourselves. We have however up to now considered such a declaration outside the range of practical politics in the United States, and the President's  views as reported in my message under reference, together with private soundings made by H.M.
Ambassador, Washington , tended to confirm this view.
2. The Netherlands Government recently communicated to the Secretary of State for Foreign Affairs here  a memorandum  suggesting that the United States Government should be approached with a view to their making clear to Japan through diplomatic channels that the United States could not tolerate continued Japanese southward aggression. The Netherlands Government also suggested that the United States Government should be informed that they themselves and the United Kingdom Government would be prepared, if the United States Government so wished, to make corresponding and simultaneous declarations to the Japanese Government. The Netherlands Government's suggestion was that these steps should not be made public for the present and in no case without previous consultation.
3. In the course of a discussion on another subject on the 5th March H.M. Ambassador at Washington mentioned to Mr. Hull  on a purely personal basis the possibility of a joint declaration by the United States, the Dutch and ourselves. Mr. Hull rather unexpectedly indicated in reply the readiness of the United States Government to do something on these lines not involving an unqualified threat. Lord Halifax feels that Mr. Hull's reply should not necessarily be taken as committing the United States Government to a joint declaration but has asked for instructions as to the manner in which he should return to the subject. 
4. Matter is at present under consideration here. ENDS.