World War I marked the first major international conflict of the twentieth century. The assassination of Habsburg Archduke Franz Ferdinand sparked the hostilities, which began in August 1914, and continued on several fronts for the next four years. During World War I, the Entente Powers—Britain, France, Serbia, and Imperial Russia (joined later by Italy, Greece, Portugal, Romania, and the United States)—fought the Central Powers—Germany and Austria-Hungary (joined later by Ottoman Turkey and Bulgaria). Initial enthusiasm faded as the war bogged down into a stalemate of costly battles and trench warfare. The system of trenches and fortifications on the Western Front extended at its longest some 475 miles. The vast expanse of the Eastern Front prevented large-scale trench warfare, but the scale of the conflict was equal to that on the West. Heavy fighting also occurred in Northern Italy, in the Balkans, and in Ottoman Turkey. Combat took place at sea and, for the first time, in the air.
World War I represented one of the most destructive wars in modern history. Nearly ten million soldiers died, a figure which far exceeded the military deaths in all the wars of the previous one hundred years combined. An estimated 21 million men were wounded in combat. The enormous losses resulted in part from the introduction of new weapons, like the machine gun and gas warfare. On July 1, 1916, the date which saw the heaviest loss of life in a single day, the British Army on the Somme alone suffered over 57,000 casualties. Germany and Russia incurred the highest number of military deaths: an estimated 1,773,700 and 1,700,000, respectively. France lost sixteen percent of its mobilized forces. Scholars estimate that as many as 13,000,000 non-combatants died as a direct or indirect result of hostilities. Mortality spiked at war's end with the outbreak of the "Spanish Flu," the deadliest influenza epidemic in history. Millions of persons were uprooted or displaced from their homes. Property and industry losses were catastrophic, especially in France and Belgium, where fighting had been the heaviest.
At 11:00 a.m. on November 11, 1918, fighting ceased on the Western Front. The "Great War," as its contemporaries called it, was over, but the conflict's far-reaching impact upon international, political, economic, and social spheres would resonate for decades to come.