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.

The Allies of World War I or Entente Powers were a coalition of countries led by France, Britain, Russia, Italy, Japan, and the United States against the Central Powers of Germany, Austria-Hungary, the Ottoman Empire, Bulgaria, and their colonies during the First World War (1914–1918).

By the end of the first decade of the 20th century, the major European powers were divided between the Triple Entente and the Triple Alliance. The Triple Entente was made up of France, Britain, and Russia. The Triple Alliance was originally composed of Germany, Austria–Hungary, and Italy, but Italy remained neutral in 1914. As the war progressed, each coalition added new members. Japan joined the Entente in 1914 and after proclaiming its neutrality at the beginning of the war, Italy also joined the Entente in 1915. The term "Allies" became more widely used than "Entente", although the Principal Allies of France, Britain, Russia, Italy, and Japan were sometimes known also as Quintuple Entente. The colonies administered by the countries that fought for the allies were also part of the Entente Powers such as British India, French Indochina, and Japanese Korea.

The United States joined in 1917 (the same year in which Russia withdrew from the conflict) as an "associated power" rather than an official ally. Other "associated members" included Serbia, Belgium, Montenegro, Asir, Nejd and Hasa, Portugal, Romania, Hejaz, Panama, Cuba, Greece, China, Siam (now Thailand), Brazil, Armenia, Guatemala, Nicaragua, Costa Rica, Haiti, Liberia, Bolivia, Ecuador, Uruguay and Honduras. The treaties signed at the Paris Peace Conference recognised Great Britain, France, Italy, Japan and the United States as 'the Principal Allied and Associated Powers'.