Answer:
Russia entered World War I in the three days succeeding July 28, 1914 — beginning with Austria-Hungary's declaration of war against Serbia, a Russian ally. Via St Petersburg, the Russian Empire sent an ultimatum to Vienna warning Austria-Hungary not to attack Serbia. Following the invasion of Serbia, Russia began to mobilize its very large reserve army. Consequently, on July 31, the German Empire in Berlin demanded Russian demobilization. There was no response; hence, on the same day, Germany declared war on Russia. In accordance with its war plan, Germany ignored Russia and moved first against France by declaring war on August 3, and by sending its main armies through Belgium to surround Paris. The threat to France caused Britain to declare war on Germany on August 4. The main belligerents had been established. (The Ottoman Empire soon joined the Central Powers and fought Russia along their border.)
Historians researching the causes of World War I have emphasised the role of Germany and Austria-Hungary. Scholarly consensus has typically minimised Russian involvement in the outbreak of this mass conflict. Key elements were Russia's defence of Orthodox Serbia, its pan-Slavic roles, its treaty obligations with France, and its concern with protecting its status as a great power. However, historian Sean McMeekin has emphasised Russian plans to expand its empire southward and to seize Constantinople as an outlet to the Mediterranean Sea.[1]
Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, was assassinated by Bosnian Serbs on 28 June 1914 due to Austria-Hungary's annexation of the largely Slavic province. Vienna was unable to find evidence that the Serbian state had sponsored this assassination but, one month later, it issued an ultimatum to Serbia, which it knew would be rejected and thus lead to war. Austria-Hungary deemed Serbia to be deserving of punishment for the assassination. Although Russia had no formal treaty obligation to Serbia, it wanted to control the Balkans, and had a long-term perspective toward gaining a military advantage over Germany and Austria-Hungary. Russia had incentive to delay militarization, and the majority of its leaders wanted to avoid war. However, Russia had the support of France and feared that a failure to defend Serbia would lead to the loss of Russian credibility, constituting a major political defeat in its goal of controlling the Balkans.[2] Tsar Nicholas II mobilized Russian forces on 30 July 1914 to threaten Austria-Hungary if it invaded Serbia. Christopher Clark stated: "The Russian general mobilisation [of 30 July] was one of the most momentous decisions of the July crisis". The first general mobilization occurred before the German government had declared a state of impending war.[3]
Germany felt threatened by Russia, responding with its own mobilization and a declaration of war on 1 August 1914. At the outset of hostilities, Russian forces led offensives against both Germany and Austria-Hungary.[4]