Gen. George C. Marshall was usually very close-mouthed about the tensions he felt with our British allies during World War II. But in a 1947 letter to McGeorge Bundy, who was helping write the memoirs of former Secretary of War Henry Stimson, Marshall was pretty candid:
... there was quite evident to me the feeling that of the British leaders that our ground army was not going to be effective, at least in time to play a decisive part in the fighting. As you know, Mr. Churchill had grave doubts about the comparative fighting ability of the American divisions as compared to the German divisions, though we should never say this publicly.... There was also the feeling that we would have no commanders comparable to the British Field Marshal..."
(P. 236, The Papers of George Catlett Marshall, Vol. 6: ‘The Whole World Hangs in the Balance.' Edited by Larry Bland, Mark Stoler, Sharon Stevens and Daniel Holt. Johns Hopkins University Press, 2013. Buy it now! )