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 t…