On the morning of September 1 the German offensive into Poland began. When Hitler spoke to the Reichstag, Germany's one-party parliament, that day, he blamed the breakdown of negotiations—in which he had refused to participate—on Poland; recounted the incidents along the border—which he had ordered staged the preceding night; and contrasted these evil deeds of others with the great generosity of his own demands—which he had carefully withheld until they had lapsed. To the thunderous applause of the representatives of the German people, he announced that Germany was once more at war.

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The French government was hearing opposite advice from its military advisors. They were concerned about air or land attacks interfering with French mobilization, and therefore wanted a maximum amount of time in order to complete as much of the mobilization process as possible before any declaration of war. Here was a divergence between the prospective allies at the very beginning of a war that was certain to strain both, one especially hard to resolve in a situation where Chamberlain faced a parliament overwhelmingly determined to move quickly—but which could hardly be enlightened about the divergent military advice received in London and Paris. Under these circumstances, the British moved ahead of the French but not so much as to make it too obvious that they were pulling a still partially reluctant French government behind them. At the last minute the expiration time of the French ultimatum to Germany was moved up twelve hours so that, although still a few hours after the British, the two declarations of war on Germany came on the same date: September 3, 1939.