The London Observer was the title given to the published version of the diaries of General Raymond E. Lee, US military attaché in London in 1940–1941, edited by James R. Leutze, which appeared in 1971. Yet Lee did not arrive in London till spring 1940, as Britain's ‘Finest Hour’ sounded and the nation squared up to Luftwaffe air attack and the threat of German invasion. Compared with this phase of the Second World War, so extensively chronicled by historians, scant attention has been paid to British preparedness for war—and especially little to evaluations by America's London observers of the quality of British preparedness. This article seeks to fill that gap. Firstly, it highlights the severe organisational shortcomings and largely absent coordination that prevented the US diplomatic and intelligence apparatus forming coherent advice to President Franklin D. Roosevelt and the State Department on whether Britain's defences, and Neville Chamberlain's government, could withstand the Third Reich. Secondly, it shows that seriously divergent assessments resulted as to the likelihood of Britain withstanding a war, notably as between the pessimism of Joseph P. Kennedy, the US ambassador to the Court of St James from 1937 to 1941 and father of later President John F. Kennedy, and the measured optimism of his professional defence advisor, Brigadier-General Bradford G. Chynoweth, Lee's predecessor as US military attaché in London. Finally, it suggests the importance of personalities in international politics, as the difficulty for the State Department, the War Department and the President of accurately calibrating British war readiness in 1939 became greatly exacerbated by the personal animosity between their men on the spot.