American foreign policy has long been dictated by the occupant of the White House. Many politicians and academics agree that no president played a higher-stakes game on the world chessboard than Franklin D. Roosevelt. While researching my book, The Ambassador, nothing shocked me more than the steadfast disagreement of Joseph P. Kennedy Sr., the American ambassador to the United Kingdom at the beginning of World War II.

During his tenure, from March 1938 to October 1940, Kennedy attempted to spearhead America’s foreign policy with the mighty and troubled British Empire. According to him, no one understood the fraught political climate in Europe better than he—not the president, and certainly not the U.S. State Department. In fact, days before he was appointed ambassador, Kennedy told the press that despite rumors of another global conflict in Europe, “the Roosevelt administration had made no clear statement of our foreign policy.” No wonder Secretary of State Cordell Hull and President Roosevelt found him insufferable from the start.

On March 11, 1938, as tensions in Europe increased, the new ambassador wrote to Roosevelt, “Nothing is going to happen.” He went on to say that Hitler’s threats were only bluffs, even though the British foreign secretary, Lord Halifax, had told Kennedy that war was inevitable.

No wonder Secretary of State Cordell Hull and President Roosevelt found Joseph P. Kennedy insufferable from the start.

That same evening, while Germany mobilized troops to take over Austria, Kennedy attended a reception in London to welcome the new dour-faced German ambassador to the U.K., Herbert von Dirksen, who had been known as the “enemy of Poland” since the 1920s. Kennedy also shook hands with the newly appointed German foreign minister, Joachim von Ribbentrop.

But by March 15, after Germany had annexed Austria, Kennedy was panic-stricken. When it came to panicking, Kennedy was a master. He phoned Hull to say that “immediate war is a greater danger than they would like to let the public know,” and that he intended to speak out against the “war-monger” Winston Churchill at a forthcoming speech. Hull warned Kennedy that it was not his job to clarify the American government’s foreign policy, and Kennedy told Hull, his boss, to “shut up.” The ambassador said that he had a much better handle on the situation.

Kennedy, a rabid isolationist from the outset, ignored cables from Vienna about the Nazi’s mass dehumanization of and violence against Jewish people, and reports about the hundreds of thousands of Jews trying to flee the Continent.

Within two weeks of arriving in London, Kennedy had declared war against the State Department and the White House. Why then did Roosevelt keep Kennedy as ambassador? As dangerous as Kennedy became in London, there is no doubt that he would have been even more divisive in Washington, D.C. Besides, Roosevelt knew that Kennedy would overplay his hand in England—which he did.

In October 1940, King George VI summed up Kennedy’s ambassadorship best: Kennedy “told Halifax that he had sent an article to the U.S. to appear in their Press on November 1, five days before the election, which would be an indictment of Roosevelt’s administration for having talked a lot and done little. He must be a very disappointed and rather embittered man, to run down his own chief.”

Susan Ronald is the author of several books, including Hitler’s Art Thief, Condé Nast, and Pirate Queen. Her latest, The Ambassador: Joseph P. Kennedy at the Court of St. James’s, 1938–1940, is out now from St. Martin’s