My Circular M.285 of 2nd September. 
As you are probably aware, the Prime Minister  in his broadcast on August 24th took the opportunity of making it clear to the public that if the efforts of the United States to reach a fair and amicable settlement with Japan should fail 'we shall, of course, range ourselves unhesitatingly at the side of United States'.
We have been giving further consideration to the question of whether it would be possible to secure from the United States some similar assurance on their part of support to us in the event of war arising between us and Japan, as a result of some action which we found it necessary to take in order to counter a further Japanese encroachment in the Far East.
It will be recalled (my Circular M-203, August 2nd ) that the observations of His Majesty's Ambassador at Washington were invited as [to] the likelihood of the United States Government being prepared to give an assurance on the above lines. In my Circular M.213, of August 5th , it was mentioned that in the view of His Majesty's Ambassador there was no reason to fear that the President and his advisers were not fully alive to the realities of the dangers to the British Empire in the Far East. It was necessary, however, to appreciate the reality of constitutional difficulties of the United States Government; if the President were to give us an assurance of support he might, in fact, be unable to implement it owing to Congressional opposition or obstruction. None the less, Lord Halifax considered that if we became involved in hostilities as a result of Japanese aggression there would, in all probability, be very great popular support in the United States for active intervention.
The matter was also discussed privately with the United States Ambassador in London  who suggested that the particular question of an assurance by the United States was one which had better be taken up directly with the President by the Prime Minister. At his meeting with President Roosevelt accordingly, the Prime Minister sounded the President as to the possibility of an intimation from him that, if any third power became the object of aggression by Japanese in consequence of counter measures which it had taken or supported to meet encroachment by Japan in the South West Pacific, he would have the intention to seek authority from Congress to give aid to such a power. President Roosevelt replied, however, without hesitation, that it was constitutionally impossible for him to go even as far as this.
In these circumstances, we feel that the matter must be left as it stands, at any rate for the present, and so far as any question of a formal assurance by the United States is concerned. You should, however, be aware that the general impression derived by our representatives at the Atlantic meeting was that, although the United States could not make any satisfactory declaration on the point, there was no doubt that in practice we could count on United States' support if, as a result of Japanese aggression, we became involved in war with Japan. In this connection, I should also invite attention to my Circular M.28  of August 12th , which indicated that the United States' Ambassador at Tokyo  had made it clear to the former Japanese Minister for Foreign Affairs  that as the United States are doing everything in their power generally to keep us supplied across the Atlantic, they obviously could not stand by and watch our lifeline being cut in the Pacific through an attack on Singapore or the Netherland East Indies.
The President assured His Majesty's Ambassador at Washington on July 31st that this would still be the United States' attitude in the event of such a Japanese attack and, as mentioned in my above telegram, Lord Halifax was later informed by the United States' Secretary of State  that this general definition of the United States' position had recently been conveyed by Welles  and himself to the Japanese Ambassador and Counsellor at Washington.