Explanation:
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
