In 1938, Roosevelt appointed Kennedy as the United States Ambassador to the Court of St. James’s (Britain). Kennedy’s Irish and Catholic status did not bother the British; indeed he hugely enjoyed his leadership position in London society, which stood in stark contrast to his outsider status in Boston. His daughter Kathleen married the heir to the Duke of Devonshire, the head of one of England’s grandest aristocratic families. Kennedy rejected the warnings of Winston Churchill that compromise with Nazi Germany was impossible; instead he supported Prime Minister Neville Chamberlain’s policy of appeasement in order to stave off a second world war that would be a more horrible “armageddon” than the first. Throughout 1938, as the Nazi persecution of Jews intensified, Kennedy attempted to obtain an audience with Adolf Hitler. Shortly before the Nazi aerial bombing of British cities began in September 1940, Kennedy sought a personal meeting with Hitler, again without State Department approval, “to bring about a better understanding between the United States and Germany.”
Kennedy argued strongly against giving aid to Britain.
“Democracy is finished in England. It may be here,” stated Ambassador Kennedy in the Boston Sunday Globe of November 10, 1940. In that one simple statement, Joe Kennedy ruined any future chances of becoming US president, effectively committing political suicide. While bombs fell daily on the UK, Nazi troops occupied Poland, Belgium, the Netherlands, and France, Ambassador Kennedy unambiguously and repeatedly stated his belief that the war was not about saving democracy from National Socialism (Nazism) or Fascism. In the now-infamous, long, rambling interview with two newspaper journalists, Louis M. Lyons of the Boston Globe and Ralph Coghlan of the St. Louis Post-Dispatch, Kennedy opined:
“It’s all a question of what we do with the next six months. The whole reason for aiding England is to give us time.” … “As long as she is in there, we have time to prepare. It isn’t that [Britain is] fighting for democracy. That’s the bunk. She’s fighting for self-preservation, just as we will if it comes to us… I know more about the European situation than anybody else, and it’s up to me to see that the country gets it,”
In British government circles during the Blitz, Ambassador Kennedy was widely disparaged as a defeatist and also known as a coward. He became known as Jittery Joe for his propensity to run for cover to an air raid shelter located near Windsor at the slightest sign of an air raid.
When the American public and Roosevelt Administration officials read his quotes on democracy being “finished”, and his belief that the Battle of Britain wasn’t about “fighting for democracy,” all of it being just “bunk”, they realized that Ambassador Kennedy could not be trusted to represent the United States. In the face of national public outcry, he was offered the chance to fall on his sword, and he submitted his resignation later that month.
Throughout the rest of the war, relations between Kennedy and the Roosevelt Administration remained tense (especially when Joe Kennedy, Jr., vocally opposed FDR’s renomination). Having effectively removed himself from the national stage, Joe Sr. sat out the war on the sidelines. Kennedy did however stay active in the smaller venues of rallying Irish and Roman Catholic Democrats to vote for Roosevelt’s reelection in 1944. He claimed to be eager to help the war effort, but as a result of his previous gaffes, he was neither trusted nor re-invited. 
With his own ambitions for the White House in self-inflicted ruins, he held out great hope for his eldest son Joseph Jr. to gain the presidency. However, Joe Jr. was killed over England while undertaking a high-risk bombing mission in 1944. Kennedy then turned his attention to grooming the second son, John F. Kennedy, who won the 1960 election.
Kennedy was (for a while) a close friend with the leading Jewish lawyer Felix Frankfurter, who helped Kennedy get his sons into the London School of Economics, where they worked with Harold Laski, a leading Jewish intellectual and prominent Socialist. While holding positive attitudes towards individual Jews, Kennedy’s views of the Jews as a people were allegedly, by his own admission, overwhelmingly negative.
According to Harvey Klemmer, who served as one of Kennedy’s embassy aides, Kennedy habitually referred to Jews as “kikes or sheenies.” Kennedy allegedly told Klemmer that “[some] individual Jews are all right, Harvey, but as a race they stink. They spoil everything they touch.” When Klemmer returned from a trip to Germany and reported the pattern of vandalism and assault on Jews by Nazis, Kennedy responded “well, they brought it on themselves.”
On June 13, 1938, Kennedy met with Herbert von Dirksen, the German ambassador in London, who claimed in Berlin that Kennedy had told him that “it was not so much the fact that we want to get rid of the Jews that was so harmful to us, but rather the loud clamor with which we accompanied this purpose. [Kennedy] himself fully understood our Jewish policy.” Kennedy’s main concern with such violent acts against German Jews as Kristallnacht was that they generated bad publicity in the West for the Nazi regime, a concern he communicated in a letter to Charles Lindbergh.
Kennedy had a close friendship with Nancy Astor; the correspondence between them is reportedly replete with anti-Semitic tropes. As Edward Renehan notes:
As fiercely anti-Communist as they were anti-Semitic, Kennedy and Astor looked upon Adolf Hitler as a welcome solution to both of these “world problems” (Nancy’s phrase)…. Kennedy replied that he expected the “Jew media” in the United States to become a problem, that “Jewish pundits in New York and Los Angeles” were already making noises contrived to “set a match to the fuse of the world.”
By August 1940, Kennedy worried that a third term for Roosevelt meant war; as Leamer reports, “Joe believed that Roosevelt, Churchill, the Jews and their allies would manipulate America into approaching Armageddon.” Nevertheless, Kennedy supported Roosevelt’s third term in return for Roosevelt’s support of Joseph Kennedy Jr. for Governor of Massachusetts in 1942.  Even during the height of the conflict, however, Kennedy remained “more wary of” prominent American Jews such as Felix Frankfurter than he was of Hitler.
Kennedy told reporter Joe Dinneen:
It is true that I have a low opinion of some Jews in public office and in private life. That does not mean that I… believe they should be wiped off the face of the earth… Jews who take an unfair advantage of the fact that theirs is a persecuted race do not help much… Publicizing unjust attacks upon the Jews may help to cure the injustice, but continually publicizing the whole problem only serves to keep it alive in the public mind.
When Dinneen wrote The Kennedy Family, he was pressured to remove these quotations from the book by John F. Kennedy himself. Dineen complied.