The gift of a common tongue is a priceless inheritance and it may well someday become the foundation of a common citizenship,” Winston Churchill prophesied in his famous speech at Harvard University on Monday, September 6, 1943. “I like to think of British and Americans moving about freely over each other’s wide estates with hardly a sense of being foreigners to one another.” His mother having been born in Brooklyn of American parentage, Churchill believed that he personified what he later called the “special relationship” between the United Kingdom and the United States. It was long a theme of his: He had been making speeches on the subject of Anglo-American unity of action since 1900, and in 1932 had signed a contract for his book A History of the English-Speaking Peoples, which emphasized the same thing.

“If we are together nothing is impossible,” he continued that day in 1943. “If we are divided all will fail. I therefore preach continually the doctrine of the fraternal association of our two peoples...for the sake of service to Mankind.” He proclaimed that doctrine for the rest of his life—indeed, on the day he resigned the premiership in April 1955 he told his cabinet, “Never be separated from the Americans.” Throughout a political career that spanned two-thirds of a century, Churchill never once publicly criticized the United States or the American people. In all of his 16 visits to the United States between 1895 and 1961, with eight as prime minister and almost half of them after 1945, he studiously confined himself to public expressions of support and approbation.

Yet as I discovered while writing my new biography, Winston Churchill: Walking With Destiny, he often took a very different stance in private. From a variety of new sources—including the wartime diaries of King George VI in the Royal Archives at Windsor Castle, opened to me by the gracious permission of Her Majesty the Queen—it is clear that Churchill regularly expressed searing criticism of the United States, and especially the administration of Franklin D. Roosevelt during World War II. The newly published diaries of Ivan Maisky, the Soviet ambassador in London from 1932 to 1943; verbatim War Cabinet records that I discovered at the Churchill Archives; and the papers of Churchill’s family, to which I have been given privileged access, all provide confirmation.

As the first Churchill biographer to be allowed to research the king’s unexpurgated wartime diaries, I was surprised at the depth of ire that Churchill sometimes directed toward Britain’s greatest ally, indeed in many ways Britain’s savior. Much can be put down to the frustration he naturally felt toward American military nonintervention in Europe until after Adolf Hitler declared war on the United States on December 11, 1941, but there was a good deal of anti-American venting thereafter, too. Churchill’s relationship with his mother country was much more complex than the Harvard speech and the rest of his public stance implied. ...