British Ambassador  saw Sumner Welles  today in continuation of our joint discussion with him (Sumner Welles) on 2nd August.
In terms which British Ambassador describes 'conversational' Sumner Welles' remarks were of first class importance and were very considerably in advance of anything that he has said before.
Sumner Welles said he was pressing the Japanese Ambassador  for a reply regarding the proposal (effectiveness of which I do not rate highly) for 'neutralizing' Thailand.
British Ambassador enquired what it was proposed to do if the neutralizing' proposal failed.
Sumner Welles said that he was today asking Counsellor of Japanese Embassy here to call on him. This particular Japanese official is just about to return to Tokyo to report on position in United States.  Sumner Welles said he proposed to tell Japanese Counsellor that 'if the Japanese pursued their projected plan in Thailand and continued on towards Netherlands East Indies or Singapore and northward (which Halifax assumed meant Burma) it was quite inevitable that, not necessarily tomorrow or next month but sooner or later, Japanese Government would find themselves involved in war with the United States.' 
Sumner Welles then went on to speak of authority given in June to American Ambassador at Tokyo  to Speak, at his discretion, in strong terms to Japanese Minister for Foreign Affairs  (referred to in my telegrams 584  and 589 ). He described potential statement (which has not, as assumed in P.M.'s  telegram 85, been made) as defining 'attitude of the United States in the event of Japanese attempt to cut the British lifeline by attack on the Netherlands East Indies and Singapore' and said, 'If the Japanese went for N.E.I. he had no doubt that that would mean war with the British Empire and that in such an event it would inevitably mean that the United States would be involved.' Sumner Welles went on to say that Japanese attack on N.E.I. or Singapore would plainly show the Japanese determination to establish a hegemony in that vital area which the United States could not tolerate. Hull  is now back at State Department after a month's illness and was to see the Thai Minister  this morning to say to him that if the Thais stood up to the Japanese, as he hoped they would, the United States would give them all the help in their power, as they had been giving help to China.