Although America has remained a neutral country for the first two years of the Second World War, the government under President Roosevelt developed a close relationship with Britain. This strained Germany’s own relationship with the USA, especially as the Navy had begun protecting merchant vessels bound for Britain by attacking German U-Boats in the Atlantic.
A week before the attack on Pearl Harbor on 7 December, the Japanese ambassador had met with the German Foreign Minister, Joachim von Ribbentrop, to secure a German commitment to war against the USA. Although this went against the terms of the Tripartite Pact that would only require German support if Japan itself was attacked, Hitler reportedly dismissed his minister’s concerns as he felt it was important to swiftly declare war on America.
Consequently Germany made plans to declare war on the USA in tandem with Italy. In a short meeting on the afternoon of 11 December Leland B. Morris, the American Chargé d'Affaires in Berlin, was summoned to Ribbentrop’s office where he was read the formal declaration of war. Half an hour later Hitler made the declaration public in an address to the Reichstag. Although he and Mussolini had agreed to announce their declarations of war at the same time, the Italian leader made his speech fifteen minutes earlier.
Thanks to the Magic system of signals intelligence that was able to decipher Japanese diplomatic communications, the United States had been aware of the German intention to declare war for two days. This gave Roosevelt time to prepare the public for an expansion of the conflict and, approximately six hours after Germany’s declaration of war, the United States declared war on Germany.
