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.
The alliance system ideated by the German chancellor Otto von Bismarck kept peace in Europe but its main aim was, however, to forestall the possibility that, in the event of war, Germany would have to fight it on two fronts (basically France and Russia).
The alliances were of vital importance for the magnitude of World War I. Even though they are not a solid reason themselves for the outbreak of war they are the key factor in the establishment of its parameters in its early days as the two fronts declared war on each other.
The alliance system was not a factor determining the initial mistrusts, disagreements and miscalculations but was the effect of such conflicts in the attempt to temporarily suppress the feeling of the inevitability of a major conflict that would eventually emerge from such quarrels.
The DualAlliance ended the isolation of France, created a rival alliance to the Triple Alliance, and, most serious of all, faced Germany with the threat of a two front war.
Secondly, the alliances were always made on a war-footing and so heightened the war tension and led to an arms race among the European powers.
The Austro-German alliance of 1879 was so modified that it had become an aggressive alliance after the Bosnian crisis in 1909, the German government promised to give military aid to Austria-Hungary, if Austria invaded Serbia and Russia intervened on behalf of the latter.