After Lend-Lease and the other support the US gave Britain I think Hitler would have eventually declared war on the US but only AFTER he decisively defeated the UK -- difficult because of places like Canada and Australia. Plans were in place to continue the British Government from Canada -- gold had been shipped there in 1939 on HMS Resolution and HMS Revenge and again these ships left Britain on May 30, 1940 with 40 million pounds in gold bullion (Operation Fish). Ostensibly this was to pay for the US for war supplies, but that Fish was more than that has come out over the years.

Hitler made his biggest mistake when he opened the Eastern Front. Stalin wanted no part of a war with Germany (although he did take his part of Poland).

Japan, short of raw materials in the home islands, should never have attacked the US and Yamamoto, who studied in the US, knew that (he had planned an attack on Pearl Harbor similar to the one executed on Dec. 7, 1941 while studying in the US). But the US Navy, like all military forces, was well equipped and ready to fight the last war all over again -- or even Trafalgar. The two most devastating blows to the Japanese navy, and Japan itself, came at the Battle of Midway and when US aircraft shot down the plane carrying Yamamoto.