The Third Battle of Ypres (German: Dritte Flandernschlacht; French: Troisime Bataille des Flandres; Dutch: Derde Slag om Ieper), also known as the Battle of Passchendaele (), was a campaign of the First World War, fought by the Allies against the German Empire. The battle took place on the Western Front, from July to November 1917, for control of the ridges south and east of the Belgian city of Ypres in West Flanders, as part of a strategy decided by the Allies at conferences in November 1916 and May 1917. Passchendaele lies on the last ridge east of Ypres, 5 mi (8.0 km) from Roulers (now Roeselare), a junction of the Bruges-(Brugge)-to-Kortrijk railway. The station at Roulers was on the main supply route of the German 4th Army. Once Passchendaele Ridge had been captured, the Allied advance was to continue to a line from Thourout (now Torhout) to Couckelaere (Koekelare).

Further operations and a British supporting attack along the Belgian coast from Nieuport (Nieuwpoort), combined with an amphibious landing (Operation Hush), were to have reached Bruges and then the Dutch frontier. Although a general withdrawal had seemed inevitable in early October, the Germans were able to avoid one due to the resistance of the 4th Army, unusually wet weather in August, the beginning of the autumn rains in October and the diversion of British and French resources to Italy. The campaign ended in November, when the Canadian Corps captured Passchendaele, apart from local attacks in December and early in the new year. The Battle of the Lys (Fourth Battle of Ypres) and the Fifth Battle of Ypres of 1918, were fought before the Allies occupied the Belgian coast and reached the Dutch frontier.

A campaign in Flanders was controversial in 1917 and has remained so. The British Prime Minister, David Lloyd George, opposed the offensive, as did General Ferdinand Foch, the Chief of Staff of the French Army. Field Marshal Sir Douglas Haig, commander of the British Expeditionary Force (BEF), did not receive approval for the Flanders operation from the War Cabinet until 25 July. Matters of dispute by the participants, writers and historians since 1917 include the wisdom of pursuing an offensive strategy in the wake of the Nivelle Offensive, rather than waiting for the arrival of the American Expeditionary Force (AEF) in France.

The choice of Flanders, its climate, the selection of General Hubert Gough and the Fifth Army to conduct the offensive, debates over the nature of the opening attack and between advocates of shallow and deeper objectives, remain controversial. The time between the Battle of Messines (714 June) and the first Allied attack (the Battle of Pilckem Ridge, 31 July), the extent to which the internal troubles of the French armies influenced the British, the effect of the exceptional weather, the decision to continue the offensive in October and the human costs of the campaign are also debated.

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.