This article lays out a 'consensus' view of Hitler's declaration of war on the US. However, it is not factually correct.
In truth, Hitler was elated by the attack on Pearl Harbor as he knew he would soon be at war with the US in any event, and a Pacific conflict would reduce the amount of effort America could direct to Europe.
It is not true that Hitler gave his advisers no warning or consulted no one. He discussed the matter with his Foreign Minister Ribbentrop and negotiated the declaration of war with his Japanese allies. He was unable to get Japan to join the war against the USSR or even blockade the port of Vladivostok but did obtain an agreement that Japan would not enter into a separate peace with Britain and the US in return for the German declaration.
On 12/11/41 Hitler knew he would be at war with the US in the very near future and declared that "if we do not join Japan in the war, the alliance is finished". Why was the US soon to be at war with Germany? Three primary reasons:
1) The Neutrality Acts had just been repealed in November. That meant that armed US merchant ships and US Navy escorts would soon be sailing all the way through the U-boat blockade into British ports. That meant either Germany had to abandon the Battle of the Atlantic or sink US ships in large numbers. The former was never considered, the latter would surely lead to a US declaration of war as it had in 1917.
2) The US was now firmly allied with Britain in a hot war with Japan. That meant FDR now had a responsibility to keep Britain in the war and the need to station US forces in England to protect his military ally from air attack. This would mean either the Germans would have to stop bombing British cities or shoot down defending US planes. The former was never considered, the latter would lead to a US declaration of war.
3) FDR's 'Victory Program' had been published by isolationist US papers a few days prior to the attack on Pearl Harbor. It specified that the US would engage in a 'Germany first' effort in and build a massive invasion army to be sent to Europe even in the event of a two-front war with Japan. Thus, Hitler knew that FDR would find a pretext to enter into a war with Germany at the earliest possible moment.
By declaring war, Hitler was able to unleash his U-boats in destructive attacks on US shipping and forced his newest enemy to direct significant effort into building naval convoy vessels instead of focusing on an invasion force and transport ships to get it to Europe.
For source notes backing up these assertions, see: "Hitler's Great Gamble" from Stackpole Books
