[2] Publicly, the formal declaration was made to American Chargé d'Affaires Leland B. Morris by German Foreign Minister Joachim von Ribbentrop in the latter's office.

This agreement, drafted on 4 December, committed the primary Axis powers to go to war with the United States in the event of hostilities with Japan, and essentially superseded the Tripartite Pact as an offensive, rather than defensive, alliance.

[7][8][9] In general, the Nazi hierarchy held low regard for the military resolve of the U.S. under Roosevelt, a stance that is widely considered a major error in their strategic thinking.

Ribbentrop telephoned the German ambassador in Rome, asking him to contact Mussolini and ensure that Italy's declaration of war be coordinated with that of Germany.

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.

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.

[24] At 3:00 pm, Hitler addressed the 855 deputies of the Reichstag gathered in the Kroll Opera House, with a speech lasting for 88 minutes in which he listed German successes to date.

[25] The second part of the speech was devoted to an attack on Roosevelt and "the Anglo-Saxon Jewish-capitalist world", concluding that "In the 2,000 years of German history known to us, our Volk has never been more unified and united than it is today".

[26] On the same day, German ambassadors in the capitals of the other Tripartite Pact signatories; Hungary, Romania, Bulgaria, Croatia and Slovakia, were instructed to obtain their declarations of war against the United States.

[23] Roosevelt had written a brief note to Congress on the morning of 11 December asking them to declare war on Germany and Italy; meeting at noon, the motion passed through both houses without dissent, although there were some abstentions.

As he said in his declaration speech to the Reichstag: I can only be grateful to Providence that it entrusted me with the leadership in this historic struggle which, for the next five hundred or a thousand years, will be described as decisive, not only for the history of Germany, but for the whole of Europe and indeed the whole world.

Jodl, who was Hitler's chief military advisor on operation planning, and his immediate second in charge, General Walter Warlimont, later recalled that it "was another entirely independent decision on which no advice from the Wehrmacht had either been asked or given".

[6][31] As early as mid-March 1941 – nine months before the Japanese attack – President Roosevelt was acutely aware of Hitler's hostility towards the United States, and the destructive potential it presented.

Due to this attitude within the White House, and the rapidly progressing efforts of the Americans' industrial capacity before and through 1941 to start providing its armed forces with the ordnance, combat aircraft and ships that would be required to defeat the Axis as a whole, the US was already well on the path to a complete war economy which would make it the "arsenal of democracy" for itself and its allies.

[32] Another factor was that Hitler's deeply-held racial prejudices made him see the US as a decadent bourgeois democracy filled with people of mixed race, a population heavily under the influence of Jews and "Negroes", with no history of authoritarian discipline to control and direct them, interested only in luxury and living the "good life" while dancing, drinking and enjoying "negrofied" music.

To Hitler the United States was a country with a white 'Nordic' racial core, to which he attributed its economic success and standard of living, and in which he saw a model for his vision of German 'living space' in Europe.

For over a year, the U.S. had been providing large amounts of economic aid to Britain and the Soviet Union in the form of loans and credit and Lend-Lease; in the Atlantic Charter, Roosevelt had pledged that America would be the "arsenal of democracy" to forestall Axis domination in Europe.

[32] From the perspective of the American administration, the United States was obliged to assist a fellow democracy in her struggle against Fascist aggression in Europe, which necessitated material and financial support, short of war, to both the British Empire and the Soviet Union.

The timing of the Japanese attack on Pearl Harbor enabled Hitler to angle his planned speech to the Reichstag in a more positive fashion, squeezing as much propaganda value out of it as possible.

Hitler, in fact, put off the speech – and the declaration of war – for several days, trying to arrive at the proper psychological moment to make the announcement.

[36] Still, the propaganda motive was hardly sufficient to justify declaring war on the US, especially considering that doing so would create an otherwise "unnatural alliance" between two disparate and heretofore antagonistic polities, the United States and the Soviet Union.

At a time when the Wehrmacht had just been forced by the Red Army and the Russian winter to move to the defense in the invasion of Russia, Hitler may have wanted to show by declaring war that he was still the master of the situation.

At least to some extent he had held in his hands the power to control the timing of the intervention of the United States, and instead, by declaring war against America, he freed Roosevelt and Churchill to act as they saw fit.

[4][8][9][37][38][39][40] From the point of view of Hitler and much of the German political and military elite, declaring war against the U.S. in response to the Pearl Harbor attack was a calculated risk in fighting the U.S. before they were prepared to effectively defend themselves.

With Nazi Germany's declaration against the United States in effect, American assistance for Britain in both theaters of war as a full ally was assured.