The Russian Empire's entry into World War I unfolded gradually in the days leading up to July 28, 1914. The sequence of events began with Austria-Hungary's declaration of war on Serbia, a Russian ally. In response, Russia issued an ultimatum to Vienna via Saint Petersburg, warning Austria-Hungary against attacking Serbia. As the conflict escalated with the invasion of Serbia, Russia commenced mobilizing its reserve army along the border of Austria-Hungary. Consequently, on July 31, Germany demanded that Russia demobilize. When Russia did not comply, Germany declared war on Russia on August 1, 1914. According to its war plan, Germany prioritized its offensive against France, declaring war on August 3. Germany deployed its main armies through Belgium with the aim of encircling Paris. The imminent threat to Belgium prompted Britain to declare war on Germany on August 4.[1][2] The Ottoman Empire subsequently joined the Central Powers and engaged in warfare against Russia along their shared border.[3]