What did Americans think of Hitler when they first met him in the 1920s and 1930s? You write that some of them burst out laughing at his shrill voice and jerky hand movements and refused to take him seriously.
That's true. In fact, some of the first people who met him did take him quite seriously. Truman Smith, who was a junior military attaché in the 1920s, came away from meeting Hitler and said, "This is a marvelous demagogue who can really inspire loyalty." It was the same with Karl von Wiegand, a Hearst correspondent who was the first American journalist to interview Hitler back in 1922. He was struck by Hitler's oratorical skills and his ability to whip people into a frenzy.
Then you had this period after the Beer Hall Putsch where Hitler came out of prison and a lot of people had forgotten about him. After the Great Depression hit, suddenly the Nazi Party became a major contender for power. Yet you had Americans meeting Hitler and saying, "This guy is a clown. He's like a caricature of himself." And a lot of them went through this whole litany about how even if Hitler got into a position of power, other German politicians would somehow be able to control him. A lot of German politicians believed this themselves.