This is my interpretation of how the Axis won WW2 in the timeline of The man in the high castle on what is mentioned. It is not the most accurate way the war could have gone but I had to fill in the missing info with the most likely results still in Axis favor.

On February 15, 1933, Franklin D. Roosevelt is assassinated by Giuseppe Zangara. Afterward, the United States is subsequently led by John Garner (FDR's vice president) and then by Republican John Bricker. These presidents led to the destabilization of the country and both failed to enact the New Deal to pull the country from the Great Depression. They continued to maintain an isolationist philosophy during the war and, as a result, they were unable to support the United Kingdom and the Soviet Union and they are unable to protect The World from the rise of the Axis threat of the Greater Nazi Reich and the Japanese Empire.