The Battle of Verdun (French: Bataille de Verdun [bataj d vd]; German: Schlacht um Verdun [laxt m vd]) was fought from 21 February to 18 December 1916 on the Western Front in France. The battle was the longest of the First World War and took place on the hills north of Verdun-sur-Meuse. The German 5th Army attacked the defences of the Fortified Region of Verdun (RFV, Rgion Fortifie de Verdun) and those of the French Second Army on the right (east) bank of the Meuse. Using the experience of the Second Battle of Champagne in 1915, the Germans planned to capture the Meuse Heights, an excellent defensive position, with good observation for artillery-fire on Verdun. The Germans hoped that the French would commit their strategic reserve to recapture the position and suffer catastrophic losses at little cost to the Germans.

Poor weather delayed the beginning of the attack until 21 February but the Germans captured Fort Douaumont in the first three days. The advance then slowed for several days, despite inflicting many French casualties. By 6 March, 20+12 French divisions were in the RFV and a more extensive defence in depth had been organised. Philippe Ptain ordered no retreat and that German attacks were to be counter-attacked, despite this exposing French infantry to German artillery-fire. By 29 March, French guns on the west bank had begun a constant bombardment of Germans on the east bank, causing many infantry casualties. The German offensive was extended to the west bank of the Meuse to gain observation and eliminate the French artillery firing over the river but the attacks failed to reach their objectives.

In early May, the Germans changed tactics again and made local attacks and counter-attacks; the French recaptured part of Fort Douaumont but then the Germans ejected them and took many prisoners. The Germans tried alternating their attacks on either side of the Meuse and in June captured Fort Vaux. The Germans advanced towards the last geographical objectives of the original plan, at Fleury-devant-Douaumont and Fort Souville, driving a salient into the French defences. Fleury was captured and the Germans came within 4 km (2 mi) of the Verdun citadel but in July the offensive was cut back to provide troops, artillery and ammunition for the Battle of the Somme, leading to a similar transfer of the French Tenth Army to the Somme front. From 23 June to 17 August, Fleury changed hands sixteen times and a German attack on Fort Souville failed. The offensive was reduced further but to keep French troops in the RFV, away from the Somme, ruses were used to disguise the change.

In September and December, French counter-offensives recaptured much ground on the east bank and recovered Fort Douaumont and Fort Vaux. The battle lasted for 302 days, the longest and one of the most costly in human history. In 2000, Hannes Heer and Klaus Naumann calculated that the French suffered 377,231 casualties and the Germans 337,000, a total of 714,231 and an average of 70,000 a month. In 2014, William Philpott wrote of 976,000 casualties in 1916 and 1,250,000 in the vicinity during the war. In France, the battle came to symbolise the determination of the French Army and the destructiveness of the war.

World War I, often abbreviated as WWI or WW1, also known as the First World War and contemporaneously known as the Great War and by other names, was an international conflict that began on 28 July 1914 and ended on 11 November 1918. It involved much of Europe, as well as Russia, the United States and Turkey, and was also fought in the Middle East, Africa and parts of Asia. One of the deadliest conflicts in history, an estimated 9 million were killed in combat, while over 5 million civilians died from occupation, bombardment, hunger or disease. The genocides perpetrated by the Ottomans and the 1918 Spanish flu pandemic spread by the movement of combatants during the war caused many millions of additional deaths worldwide.In 1914, the Great Powers were divided into two opposing alliances: the Triple Entente, consisting of France, Russia, and Britain, and the Triple Alliance, made up of Germany, Austria-Hungary, and Italy. Tensions in the Balkans came to a head on 28 June 1914 following the assassination of Archduke Franz Ferdinand, the Austro-Hungarian heir, by Gavrilo Princip, a Bosnian Serb. Austria-Hungary blamed Serbia and the interlocking alliances involved the Powers in a series of diplomatic exchanges known as the July Crisis. On 28 July, Austria-Hungary declared war on Serbia; Russia came to Serbia's defence and by 4 August, the conflict had expanded to include Germany, France and Britain, along with their respective colonial empires. In November, the Ottoman Empire, Germany and Austria formed the Central Powers, while in April 1915, Italy joined Britain, France, Russia and Serbia as the Allied Powers.

Facing a war on two fronts, German strategy in 1914 was to defeat France, then shift its forces to the East and knock out Russia, commonly known as the Schlieffen Plan. This failed when their advance into France was halted at the Marne; by the end of 1914, the two sides faced each other along the Western Front, a continuous series of trench lines stretching from the Channel to Switzerland that changed little until 1917. By contrast, the Eastern Front was far more fluid, with Austria-Hungary and Russia gaining, then losing large swathes of territory. Other significant theatres included the Middle East, the Alpine Front and the Balkans, bringing Bulgaria, Romania and Greece into the war.

Shortages caused by the Allied naval blockade led Germany to initiate unrestricted submarine warfare in early 1917, bringing the previously neutral United States into the war on 6 April 1917. In Russia, the Bolsheviks seized power in the 1917 October Revolution and made peace in the March 1918 Treaty of Brest-Litovsk, freeing up large numbers of German troops. By transferring these to the Western Front, the German General Staff hoped to win a decisive victory before American reinforcements could impact the war, and launched the March 1918 German spring offensive. Despite initial success, it was soon halted by heavy casualties and ferocious defence; in August, the Allies launched the Hundred Days Offensive and although the German army continued to fight hard, it could no longer halt their advance.Towards the end of 1918, the Central Powers began to collapse; Bulgaria signed an Armistice on 29 September, followed by the Ottomans on 31 October, then Austria-Hungary on 3 November. Isolated, facing revolution at home and an army on the verge of mutiny, Kaiser Wilhelm abdicated on 9 November and the new German government signed the Armistice of 11 November 1918, bringing the fighting to a close. The 1919 Paris Peace Conference imposed various settlements on the defeated powers, the best known being the Treaty of Versailles. The dissolution of the Russian, German, Ottoman and Austro-Hungarian empires led to numerous uprisings and the creation of independent states, including Poland, Czechoslovakia and Yugoslavia. For reasons that are still debated, failure to manage the instability that resulted from this upheaval during the interwar period ended with the outbreak of World War II in 1939.