I see that some people in Britain are in a bit of a kerfuffle about recent indications that the Americans would not be pleased if they left the EU. So it seems appropriate to quote at length from a well-known passage by Miriam Camps (1964, pp. 336-7):
Early in April , Mr. Macmillan went to Washington for talks with the new Administration. Although he had met the new President at Palm Beach in connexion with the Laos crisis, the April visit was the first opportunity for a general review of common problems, and Britain’s relations with the Common Market was obviously one of the matters which Mr. Macmillan wanted to discuss. The available evidence suggests that Mr. Macmillan asked Mr. Kennedy a hypothetical question: ‘What would be your reaction if we decided to join the EEC?’ and that he was given an enthusiastic affirmative answer. There is no evidence to suggest that Mr. Macmillan was ‘pushed’ by Mr. Kennedy, as was alleged, and denied, at various times. But it is clear that Mr. Kennedy left no doubt in Mr. Macmillan’s mind that a British decision to join the Six would be welcome and that Mr. Macmillan left Washington convinced that, far from straining Anglo-American relations, Britain’s joining the Community might well lead to much closer and more far-reaching transatlantic links than the British could hope to achieve in other ways. The reflection that the shortest, and perhaps the only, way to a real Atlantic partnership lay through Britain’s joining the Common Market seems to have been a very important — perhaps the controlling — element in Mr. Macmillan’s own decision that the right course for the United Kingdom was to apply for membership. Mr. Kennedy’s warm response undoubtedly strengthened Mr. Macmillan’s own conviction that joining was the right course of action and encouraged him to continue his efforts to bring the sceptics in the Cabinet to accept this view. Also, like the discussions with General de Gaulle and Dr. Adenauer earlier in the year, the discussions with the United States Administration underlined, once again, the fact that ‘association’ arrangements were not likely to be negotiable. It was clear that the United States was prepared to accept the additional commercial ‘discrimination’ against itself because of the political advantages it saw in British membership in the Community, but that it would be hostile to arrangements short of membership which, in its view, would simply increase ‘discrimination’ but would not, like full membership, add to the political stability of the Community or strengthen the ‘Atlantic’ orientation of the new power-complex the Six were clearly coming to be.