The Russian Empire gradually entered World War I during the three days prior to July 28, 1914. This began with Austria-Hungary's declaration of war against Serbia, which was a Russian ally at the time. The Russian Empire sent an ultimatum, via St Petersburg, to Vienna, warning Austria-Hungary not to attack Serbia. Following the invasion of Serbia, Russia began to mobilize its reserve army near its border with Austria-Hungary. Consequently, on July 31, the German Empire in Berlin demanded Russian demobilization. There was no response, which resulted in the German declaration of war on Russia on the same day (1 August, 1914). In accordance with its war plan, Germany disregarded Russia and moved first against France, declaring war on August 3rd. Germany sent its main armies through Belgium to surround Paris. The threat to Belgium caused Britain to declare war on Germany on August 4. The Ottoman Empire soon after joined the Central Powers and fought Russia along their border.