Dual Alliance, 1879
The Dual Alliance was created by the treaty of October 7th, 1879, by which Germany and Austria-Hungary undertook to support one another if either was attacked by Russia, and to offer each other benevolent neutrality in the event of attack by another power, generally (and especially following the Franco-Russian alliance of 1891-94) taken to represent France.
The Alliance's existence was the reason for Russia's military mobilisation (July 30, 1914) against both allies (despite the absence of an immediate Russo-German dispute), rather than against Austria-Hungary alone following the latter's declaration of war against Serbia, whose independence Russia had pledged to uphold.
Russia found it impossible to comply with Germany's subsequent demand that she stand down her forces, since her war plan had been drawn up with both allies in mind, and mobilisation against Germany could not be countermanded without jeopardising preparations against Austria-Hungary also.
Germany's resulting declaration of war against Russia (August 1) and France (August 3) signalled the general outbreak of World War I among Europe's Great Powers, though by August 4, with all engaged in hostilities apart from neutral Italy (nominally joined with Germany and Austria-Hungary in the Triple Alliance), none had actually declared war in fulfillment of an alliance obligation.