A Serbian nationalist assassinated the archduke and heir to the throne of Austria-Hungary in late June of 1914 — an act widely considered the spark that ignited World War I. In the aftermath of the assassination, Austria-Hungary, backed by Germany, presented Serbia with an ultimatum that laid out ten specific demands. Among them: Serbia was to suppress rhetoric and media expressing contempt for the Austro-Hungarian monarchy, disband Serbian nationalist organizations, and dismiss administration and military officials connected to anti-Austrian activities. While Serbia agreed to meet most of these demands, it stopped short of full compliance and refused to take part in a complete investigation into the archduke's assassination. That refusal became a flashpoint between the two nations. The dispute culminated in Austria-Hungary declaring war on Serbia on July 28, 1914 — a moment that marked the official beginning of World War I.
What started as a conflict between two nations spiraled rapidly, pulled outward by an intricate web of alliances. Russia moved to mobilize its troops in defense of its ally, Serbia, as soon as Austria-Hungary's declaration of war became official. Germany, acting on behalf of Austria-Hungary, responded by declaring war on Russia. The fighting then spread further when Germany declared war on France, which was allied with Russia. Germany's invasion of Belgium drew Great Britain into the war as well. In the span of just a few months following these declarations, the conflict had consumed much of Europe.
The fighting that defined World War I stretched on for four years before finally coming to a close in 1918. On November 11, 1918, Germany and the Allied powers signed an armistice that brought an end to much of the combat — though it fell short of being a formal German surrender. Peace treaties, most notably the Treaty of Versailles, officially concluded the war. Yet even though the Treaty of Versailles may have ended World War I, it hardly ushered in lasting peace across Europe. The harsh penalties it imposed on Germany fueled instability throughout the region, and the lingering hostilities and tensions would eventually give way to even greater conflicts.