Russia entered into World War I on July 28, 1914, when the Austro-Hungarian Empire declared war on Russia's ally Serbia.
Russia sent an ultimatum to Austria-Hungary stating, if Austria-Hungary showed aggression toward its allies, they would meet with military action.
Once Austria-Hungary declared war on Russia Germany followed suit. In accordance with its war plan, Germany ignored Russia and moved first against France—declaring war on August 3 and sending its main armies through Belgium to attack Paris from the north.
The invasion of Belgium caused Britain to declare war on Germany on August 4.
The main parties were now at war. Soon Turkey joined on Germany's side, and later Italy on the Allied side.
Historians on the causes of World War I have emphasized the role of Germany and the Austro-Hungarian Empire.
The consensus of scholars includes scant mention of Russia, and briefly mentions Russia's defense of Orthodox Serbia, its pan-Slavic roles, its treaty obligations with France, and its concern for protecting it status as a great power.
The Archduke Franz Ferdinand of Austro-Hungary was assassinated by Bosnian Serbs on 28 June 1914 to protest Austrian takeover of a largely Slavic province.
Vienna was unable to find evidence that Serbia had sponsored the assassination but a month later issued an ultimatum to Serbia, which it knew would be rejected and thus lead to war.
Austria-Hungary felt that Serbia had to be destroyed or else it would destabilize Austria by alienating Austria's very large Slavic minority element.
Russia did not have a treaty obligation to Serbia but stood in opposition to Austria for control of the Balkans.
In long-term perspective, Russia was militarily gaining on Germany and Austro-Hungary, and thus had an incentive to wait.
Most Russian leaders wanted to avoid a war.
However, in the present crisis they had the support of France, and they feared that failure to support Serbia would lead to a loss of Russian credibility and a major political defeat to Russia's goals for a leadership role in the Balkans.
Tsar Nicholas II mobilised Russian forces on 30 July 1914 to threaten Austria if it invaded Serbia.
Christopher Clark states: "The Russian general mobilisation [of 30 July] was one of the most momentous decisions of the July crisis."
This was the first of the general mobilisations.
It came at the moment when the German government had not yet even declared the State of Impending War".
Germany now felt threatened by Russia and responded with her own mobilisation and declaration of war on 1 August 1914. At the opening of hostilities, the Russians took the offensive against both Germany and Austria-Hungary