It is over thirty years since the U. S. became embroiled in the Vietnam war. Only recently, however, have scholars begun to assess how that involvement in South East Asia affected America's relations with other countries. This thesis examines the impact of the Vietnam war on the relationship between the United States and one of its key allies, Great Britain, during the height of the conflict. It assesses how far Vietnam was a factor in the cooling of transatlantic relations during the mid to late 1960s. Scholars have long noted the decline in importance of Anglo-American relations during the 1960s. It is the contention of this thesis that the Johnson administration's preoccupation with events in South East Asia made the inevitable loosening of ties between the two countries strained and uncomfortable. Although it was not the only problematic issue troubling Anglo-American relations during this period, Vietnam was the one area where there was clear and open conflict. Whereas tensions over sterling and the decision by the British Government to remove its troops from East of Suez prompted feelings of disappointment, sadness and frustration, Vietnam provoked disagreement, misunderstandings, annoyance and accusations of betrayal. At the beginning of their period in office, the British Labour Government desired a `closer' relationship with the United States but by 1968 it was apparent that the Johnson Administration was not amenable to this. This was partly because Britain was now, just one of a number of close allies in Europe; partly because the American President did not develop a personal friendship with the British Prime Minister; but also because the Vietnam conflict had proved an issue - important enough and emotive enough - to cause open and deep disagreement between the two countries.