Russia entered World War I three days after July 28, 1914, when Austria-Hungary's declared war on Serbia, a Russian ally. The Russian Empire sent an ultimatum warning Austria-Hungary not to attack Serbia, but after the invasion of Serbia, Russia began to mobilize its army. So, on July 31, the German Empire demanded Russian demobilization. Russia didn’t respond, so Germany declared war on Russia. Image: Russian recruiting poster reads 'World on fire; Second Patriotic War'.