The war's trigger was the assassination of Archduke Franz Ferdinand of Austria-Hungary in Sarajevo on 28 June 1914 by a Serb nationalist. The real causes were far deeper: imperial rivalries, an arms race, entangled alliances, nationalist tensions, and the vacuum left by Ottoman decline. Within weeks what had begun as a Balkan crisis became a continental and then a global conflict.

The Western Front froze for years in muddy trenches across Belgium and northern France. Battles like the Somme (1916), Verdun (1916), and Passchendaele (1917) cost hundreds of thousands of lives without gaining meaningful ground. On the Eastern Front, Germany ground Russia down until the 1917 revolution. For the Ottomans, Gallipoli (1915), Kut, and the Palestine campaigns were decisive — and the Arab Revolt together with the secret Anglo-French Sykes–Picot agreement of 1916 seeded the boundaries of the modern Middle East.

The war was the first conducted at full industrial scale: machine guns, heavy artillery, submarines, tanks, chemical weapons, aircraft. Civilian deaths approached military casualties; war-driven catastrophes including the 1915 Armenian deportations and massacres and the 1918 Spanish flu brought total deaths within the decade to around 100 million. Four empires collapsed — German, Austro-Hungarian, Russian, Ottoman. The harsh terms imposed on Germany at Versailles (1919) helped set the conditions for the Second World War within twenty years.