The Austrians decided to present an unacceptable ultimatum to Serbia and then to declare war, relying on Germany to deter Russia from intervention. Though the terms of the ultimatum were finally approved on July 19, its delivery was postponed to the evening of July 23, since by that time the French president, Raymond Poincaré, and his premier, René Viviani, who had set off on a state visit to Russia on July 15, would be on their way home and therefore unable to concert an immediate reaction with their Russian allies. When the delivery was announced, on July 24, Russia declared that Austria-Hungary must not be allowed to crush Serbia.
Serbia replied to the ultimatum on July 25, accepting most of its demands but protesting against two of them—namely, that Serbian officials (unnamed) should be dismissed at Austria-Hungary’s behest and that Austro-Hungarian officials should take part, on Serbian soil, in proceedings against organisations hostile to Austria-Hungary. Though Serbia offered to submit the issue to international arbitration, Austria-Hungary promptly severed diplomatic relations and ordered partial mobilisation.
Home from his cruise on July 27, William learned on July 28 how Serbia had replied to the ultimatum. At once he instructed the German Foreign Office to tell Austria-Hungary that there was no longer any justification for war and that it should content itself with a temporary occupation of Belgrade. But, meanwhile, the German Foreign Office had been giving such encouragement to Berchtold that already on July 27 he had persuaded Franz Joseph to authorise war against Serbia. War was in fact declared on July 28, and Austro-Hungarian artillery began to bombard Belgrade the next day. Russia then ordered partial mobilisation against Austria-Hungary, and on July 30, when Austria-Hungary was riposting conventionally with an order of mobilisation on its Russian frontier, Russia ordered general mobilisation. Germany, which since July 28 had still been hoping, in disregard of earlier warning hints from Great Britain, that Austria-Hungary’s war against Serbia could be “localised” to the Balkans, was now disillusioned insofar as eastern Europe was concerned. On July 31 Germany sent a 24-hour ultimatum requiring Russia to halt its mobilisation and an 18-hour ultimatum requiring France to promise neutrality in the event of war between Russia and Germany.
Both Russia and France predictably ignored these demands. On August 1 Germany ordered general mobilisation and declared war against Russia, and France likewise ordered general mobilisation. The next day Germany sent troops into Luxembourg and demanded from Belgium free passage for German troops across its neutral territory. On August 3 Germany declared war against France.
In the night of August 3–4 German forces invaded Belgium. Thereupon, Great Britain, which had no concern with Serbia and no express obligation to fight either for Russia or for France but was expressly committed to defend Belgium, on August 4 declared war against Germany.
Austria-Hungary declared war against Russia on August 5; Serbia against Germany on August 6; Montenegro against Austria-Hungary on August 7 and against Germany on August 12; France and Great Britain against Austria-Hungary on August 10 and on August 12, respectively; Japan against Germany on August 23; Austria-Hungary against Japan on August 25 and against Belgium on August 28.
Romania had renewed its secret anti-Russian alliance of 1883 with the Central Powers on February 26, 1914, but now chose to remain neutral. Italy had confirmed the Triple Alliance on December 7, 1912, but could now propound formal arguments for disregarding it: first, Italy was not obliged to support its allies in a war of aggression; second, the original treaty of 1882 had stated expressly that the alliance was not against England.