Why it started
The immediate trigger was the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by the Bosnian Serb nationalist Gavrilo Princip in Sarajevo on 28 June 1914. Austria-Hungary blamed Serbia. The system of pre-existing European alliances — the Triple Entente (France, Britain, Russia) and the Triple Alliance (Germany, Austria-Hungary, Italy) — turned a Balkan crisis into a continental war within five weeks.
The deeper causes included: industrialized European nationalism; the rise of Germany since unification in 1871 and the resulting shift in the European balance of power; the long-running competition for colonial territories; the naval arms race between Britain and Germany; the systematic militarization of railway timetables, which made any partial mobilization politically and operationally difficult to reverse; and several specific diplomatic miscalculations in July and August 1914.
How it was fought
The war was the first industrial war fought at continental scale. The Western Front (in France and Belgium) settled into trench warfare by late 1914 and remained largely static for the next four years. New weapons appeared at unprecedented scale: machine guns, heavy artillery, aircraft (initially for reconnaissance, later for combat), submarines, poison gas (first used at Ypres, April 1915), and tanks (first used at the Somme, September 1916).
The Eastern Front (Russia against Germany and Austria-Hungary) was more mobile but also produced massive casualties. The Ottoman fronts (against Russia in the Caucasus, against Britain in Mesopotamia, Palestine, and Gallipoli) involved substantial fighting in difficult terrain. There were also smaller campaigns in Africa, in the Pacific, and at sea.
The single bloodiest battles were the Somme (July–November 1916, ~1.2 million casualties), Verdun (February–December 1916, ~700,000 casualties), and the third battle of Ypres / Passchendaele (July–November 1917, ~500,000 casualties).
Turning points
The Russian Revolution of 1917 took Russia out of the war by November 1917. The United States declared war on Germany on 6 April 1917, following the German resumption of unrestricted submarine warfare. American troops and matériel began arriving in France in significant numbers by mid-1918.
The German Spring Offensive of March–July 1918 was the last German attempt at decisive victory and failed. The Allied counterattack from August 1918 (the Hundred Days Offensive) pushed German forces back. Germany’s allies collapsed: Bulgaria signed an armistice on 29 September; the Ottoman Empire on 30 October; Austria-Hungary on 3 November. Germany itself signed the armistice in a railway carriage at Compiègne, France, at 11 a.m. on 11 November 1918.
Consequences
The Treaty of Versailles (28 June 1919) imposed substantial reparations on Germany, restricted the German military, and assigned German colonies to the victorious powers. The German monarchy had already collapsed; Austria-Hungary disintegrated into Austria, Hungary, Czechoslovakia, Yugoslavia, and parts of Italy, Poland, and Romania; the Ottoman Empire collapsed and was partitioned, becoming the Republic of Turkey plus French, British, and other mandates across the Middle East. The Russian Empire became the Soviet Union.
The war’s death toll, financial costs, political consequences, and accumulated trauma shaped European and global history for the rest of the 20th century — including, by reasonable causal argument, the conditions that produced World War II two decades later. The Halifax Explosion of December 1917 — a wartime munitions accident — was one of the largest non-combat events of the war.