In addition to the answers already written, I’d like to add that Hitler had a childish and romantic idea of America based on the Western novels by the German writer Karl May. These books were very popular in Germany in the first half of the 20th century and featured the exploits of amongst others, “Old Shatterhand”.
Hitler saw America as an underdeveloped frontier much as it might have been at the time of the novels. Perhaps this allowed him to grossly underestimate the consequences of his declaration of war just after Pearl Harbour.