The Serbian Expansion and Russian role in the Balkans.
Combined with the increase in Russian military strength, both Austria and Germany felt threatened by Serbian expansion; when Austria invaded Serbia on 28 July 1914, Russian Foreign Minister Sergey Sazonov viewed it as an Austro-German conspiracy to end Russian influence in the Balkans.
On 30 July, Russia declared general mobilization in support of Serbia; on 1 August, Germany declared war on Russia, followed by Austria-Hungary on 6th.