The Austrio-Hungarian Council of Ministers met in secret, ultimately voting to declare war on Serbia. On the one hand, this seemed a senseless decision: why declare war when your (Austria’s) army was still more than two weeks from completing its mobilization process? On the other hand, Austro-Hungarian leaders knew that in order to punish Serbia properly, they needed the support of Germany to stave off Russia, and though Germany had promised its support, an early declaration of war helped to commit them to this position before they had an opportunity to reconsider.
This became important when, later that day, the text of Serbia’s response to Austria-Hungary’s ultimatum became available. British foreign secretary Edward Grey saw the Serbian response as an immense capitulation, and saw an opportunity for a negotiated settlement among the nations of the area. When Germany’s Kaiser Wilhelm returned from his vacation and read Grey’s assessment the next day before also reading the Serbian response, he would indeed attempt to soften Germany’s support for Austria, calling the Serbian response “a capitulation of the most humiliating kind,” so that “every cause for war falls to the ground!”
~G.J. Meyer, A World Undone: The Story of the Great War, 1914-1918 (New York: Delacorte Press, 2006), 49-53.
The early and seemingly unnecessary declaration of war upon Serbia therefore served to fix both Austria and Germany into a course towards war against Serbia. Mobilizing an army was expensive, and Austria-Hungary’s previous two mobilizations, during the Balkan wars earlier in the 1910s, had served no purpose at all, the army standing idle as other nations gained the spoils of war. This time, Austria-Hungary wanted its money’s worth.