The outbreak of the First World War did not occur all at once. Rather, the war started over the course of a month as nations mobilized and declared war on each other in turn. One of these steps was the German declaration of war against Russia on August 1st.
Germany was obliged to declare war on Russia by the alliance that it formed before the war. When Austria declared war on Serbia following the assassination of Archduke Franz Ferdinand, Russia mobilized and declared war on Austria. In response, German mobilized to defend its ally and declared war on August 1st. However, Germany’s initial efforts during the war were focused on France rather than Russia. Russia was a larger country with a more dispersed industry than France. German generals thus believed that France could be quickly forced out of the war, while the Russian Army would take a long time to mobilize and thus could not invade Germany. This plan, dubbed the Schlieffen Plan after its creator, was activated on the day of the declaration and two days later Germany declared war on France. The invasion of France went well initially, but at the Battle of the Marne, the Germans were stopped. The Russian Army mobilized far more quickly than was expected and soon invaded East Prussia. However, the outnumbered Germans, reinforced by some troops from the Western Front, trounced the poorly-led Russians at the Battle of Tannenberg. The war in the east was very mobile, and the Russians were pushed far back from their borders. Although they had some successes against Austria, by 1917 Russia was in crisis as its population no longer supported the war and its army was in tatters. The Russian Revolution that year effectively ended Russian action against Germany, and that year the Treaty of Brest-Litovsk saw Russia surrender.
The German declaration of war on Russia should not have happened. The alliance system that made a war in the Balkans a global war was responsible for the deaths of millions. The war destroyed both the German and Russian empires, and the rulers who made those alliances would either be executed or die in exile. While isolation from foreign affairs is foolish, nations should be careful with choosing their allies, and what commitment they guarantee them.