In some of my other posts on this forum, I’ve described how even in 1940 (when the U.S. was still technically at peace with Germany), it had devoted a significant amount of industrial potential toward winning the air war against Germany; with plans to considerably expand the effort over the next several years. America’s strongest single asset was her industrial potential, as Hitler clearly understood. But that potential was going to be increasingly turned against Germany, whether the U.S. was technically at war or not.
Hitler’s plan to counter this was to expand Germany’s industrial output over the short-term, so that he could at least keep pace with the air war over Germany in the long haul. His method of expanding Germany’s output included industrialization and conquest. The industrialization aspect of his plan meant that instead of putting everything he had into weapons output for 1941 or 1942–in a massive effort to crush the Soviet Union–he had to divide his nation’s economic activity between short-term military production and long-term output increases. The result of all that industrial investment was that Germany increased its aircraft output from 16,000 planes per year in 1941 to over 40,000 planes per year by 1944. It also increased its military production in other categories, such as tanks and V2 rockets.
In late 1941, Hitler knew that his window of opportunity to win the war was relatively slim; and that 1942 would be critical. Germany had to win a decisive victory over the Soviet Union in that year, both to take pressure off its eastern front, and to provide it with the raw materials and manpower it needed to hold its own in the air war over the long haul. Hitler believed that the overwhelming majority of America’s naval strength would be needed in the Pacific to counter the Japanese; at least until the end of 1943. That gave him what he believed was a two year window with which he could sink the American Lend-Lease Aid pouring into Great Britain and the Soviet Union. Sinking those ships would increase Germany’s chances of obtaining the victory over the Soviet Union it desperately needed.
However, the Japanese Navy proved less adept than Hitler had hoped. The Battle of Midway occurred six months after Pearl Harbor. That battle took the naval pressure off the U.S. in the Pacific, and allowed it to focus more of its efforts on convoy protection in the Atlantic. More generally, the failure of the Japanese military meant that Japan would be far less successful in taking military pressure off of Germany than Hitler had hoped.
In 1942, the Soviet Union outproduced Germany 3:1 - 4:1 in tanks, artillery, and other land combat categories, and even 2:1 in military aircraft. It also fielded a much larger army than Germany. Thus evaporated Hitler’s hope of a decisive victory on his eastern front.
Had Hitler not declared war against the U.S., large amounts of American Lend-Lease aid would still have flowed to his enemies in Europe. Over the long-term, he still would have needed to devote a significant portion of his military production to defending German skies and German cities against American-made bombers. He would have lost out on the Second Happy Time (see http://en.wikipedia.org/wiki/Second_Happy_Time ), which would have resulted in a stronger Soviet war effort for 1942. On the other hand, he wouldn’t have had to deal with the U.S. invasion of Algeria in 1942, its invasion of Sicily and Italy in 1943, or the Normandy invasion of '44. It’s easy to say in hindsight that the harm of the American invasions exceeded the benefit of sinking that Lend-Lease shipping in '42. But that distinction was less obvious at the time.