Japan, Germany and Italy officially became allies after signing the Tripartite Pact of 1940.
Article 3 covered their obligations in case of war (emphasis mine):
ARTICLE 3. Japan, Germany, and Italy agree to cooperate in their efforts on aforesaid lines. They further undertake to assist one another with all political, economic and military means if one of the Contracting Powers is attacked by a Power at present not involved in the European War or in the Japanese-Chinese conflict.
IOW, Germany ONLY had to declare war on the U.S. if it attacked Japan. But Japan attacked us first. Germany was under absolutely no obligation to declare war on the U.S.
I've always wondered why Germany did. By December 1941, Hitler had a long history of tearing up treaties/agreements whenever it suited him. Declaring war on the U.S. was one of the two things that guaranteed Nazi Germany would lose the war...the other being its invasion of Russia in June 1941. Which, er, um, oh yes...was pretty obviously a violation of the Russo-German Nonaggression Pact of August 1939.
The History Channel recently ran a documentary on the Pearl Harbor conspiracy theories. As one historian pointed out, these theories are almost always politically driven. Specifically by right-wing cranks.
FWIW, the official German declaration of war on the U.S. doesn't even mention Japan. This is the whole thing:
German Declaration of War against the United States
The Government of the United States having violated in the most flagrant manner and in ever-increasing measure all rules of neutrality in favor of the adversaries of Germany and having continually been guilty of the most severe provocations toward Germany ever since the outbreak of the European war, provoked by the British declaration of war against Germany on September 3, 1939, has finally resorted to open military acts of aggression.
On September 11, 1941, the President of the United States publicly declared that he had ordered the American Navy and Air Force to shoot on sight at any German war vessel. In his speech of October 27, 1941, he once more expressly affirmed that this order was in force. Acting under this order, vessels of the American Navy, since early September 1941, have systematically attacked German naval forces. Thus, American destroyers, as for instance the Greer, the Kearny and the Reuben James, have opened fire on German submarines according to plan. The Secretary of the American Navy, Mr. Knox, himself confirmed that American destroyers attacked German submarines.
Furthermore, the naval forces of the United States, under order of their Government and contrary to international law have treated and seized German merchant vessels on the high seas as enemy ships.
The German Government therefore establishes the following facts:
Although Germany on her part has strictly adhered to the rules of international law in her relations with the United States during every period of the present war, the Government of the United States from initial violations of neutrality has finally proceeded to open acts of war against Germany. The Government of the United States has thereby virtually created a state of war.
The German Government, consequently, discontinues diplomatic relations with the United States of America and declares that under these circumstances brought about by President Roosevelt, Germany too, as from today, considers herself as being in a state of war with the United States of America.
Accept, Mr. Chargé d'Affaires, the expression of my high consideration.
December 11, 1941
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