ORIGINAL: Zovs
On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, on 23 July Austria-Hungary issued an ultimatum to Serbia. Serbia's reply failed to satisfy the Austrians, and the two moved to a war footing.
A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe. By July 1914, the great powers of Europe were divided into two coalitions: the Triple Entente—consisting of France, Russia, and Britain—and the Triple Alliance of Germany, Austria-Hungary, and Italy (the Triple Alliance was primarily defensive in nature, allowing Italy to stay out of the war in 1914). Russia felt it necessary to back Serbia and, after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, approved partial mobilisation. Full Russian mobilisation was announced on the evening of 30 July; on the 31st, Austria-Hungary and Germany did the same, while Germany demanded Russia demobilise within twelve hours. When Russia failed to comply, Germany declared war on Russia on 1 August in support of Austria-Hungary, with Austria-Hungary following suit on the 6th; France ordered full mobilisation in support of Russia on 2 August.
So, the first belligerent act was by Serbia, and the first Major Power to trip the alliance dominoes was Russia. Yet the Central Powers get the blame for some reason.