The Battle of Passchendaele, one of the longest, bloodiest and most controversial battles of World War One, began 100 years ago. Also known as the third battle of Ypres, the campaign was launched on 31 July 1917 and immediately became bogged down as heavy rain turned the Flanders lowlands into a mud-churned swamp rendering tanks immobile and virtually paralysing the infantry.

The Allied forces – made up troops from Britain, France, Belgium and the British Empire (including Australia, Canada, New Zealand, South Africa and India) – aimed to capture the ridges south and east of Ypres on the Western Front from the German Empire.

Any gains they made were almost invariably soon lost, and the battle became a war of attrition. As historian Richard van Emden writes in his book The Road to Passchendaele: "1917 [was] a year of unparalleled misery on both sides of the line: an end to the war was nowhere near in sight, and popular enthusiasm for the struggle had long since eroded. The struggles ... were characterised by a grim resignation to the necessity of attrition and gradual battlefield predominance in men and arms. All in all, 1917 completed the transition to the 'wearing out' war."

The battle finally ended in appalling weather and with demoralised and tired troops after village of Passchendaele was seized by Canadian infantry in November 1917. Casualty figures are disputed, but it is thought around 310,000 Allied soldiers and 260,000 German soldiers lost their lives at Passchendaele.

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31 July 1917: Three British guardsmen look at the body of a dead German in a shell hole after the Battle of Pilckem RidgeHulton Archive/Getty Images
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A stretcher-bearing party carry a wounded soldier through the mud near Boesinghe during the battle of Passchendaele in FlandersJohn Warwick Brooke/Getty Images
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31 July 1917: A German two seater Fokker Wolf, shot down on the first day of the Battle of Pilckem RidgeHulton Archive/Getty Images
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16 August 1917: A 6-inch howitzer of the Royal Garrison Artillery is pulled into position past some ruins near Boezinge ahead of the Battle of LangemarckIWM
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16 August 1917: German prisoners sprawled exhausted on the ground near ProvenIWM
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17 August 1917: A shell bursts in the ruins of Bossinghs Station, three miles north of Ypres, during the Battle of LangemarckHulton Archive/Getty Images
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September 1917: A British tank crosses the trenches in FlandersHulton Archive/Getty Images
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Wreckage of a German Albatross D III fighter biplaneUS Library of Congress
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Abandoned British trench which was captured by the GermansUS Library of Congress
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German prisoners are marched thru Ypres after the the Battle of Menin RoadUS Library of Congress
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Wounded British prisoners are escorted on the Menin Road by a German soldierHulton Archive/Getty Images
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Australian walking wounded on the Menin Road return from the frontHulton Archive/Getty Images
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27 September 1917: Australian infantry from the 45th Battalion, Australian 4th Division at Garter Point near Zonnebeke, Ypres sectorCaptain Frank Hurley
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5 October 1917: Supporting troops of the 1st Australian Division walk towards the front line to relieve their comrades, whose attack the day before won Broodseinde RidgeFrank Hurley/Hulton Archive/Getty Images
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10 October 1917: An 18-pounder being hauled through the mud at Broodseinde Ridge in support of the advancing AustraliansTasmanian Archive
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12 October 1917: A New Zealand signaller sends messages to advanced troops from a German dug-out at Gallipoli Farm, a main German strongpoint during the Third battle of Ypres (Passchendaele)Henry Armytage Sanders
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29 October 1917: Soldiers of an Australian 4th Division field artillery brigade walk on a duckboard track passing through Chateau Wood, near HoogeCaptain Frank Hurley
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A British soldier advances under cover of what remains of a tree-lined road, on the approach to YpresHulton Archive/Getty Images
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Late October or early November 1917: Troops of the Canadian 16th Machine Gun Company hold the line in atrocious conditions on the Passchendaele front. Corporal Ronald Lebrun, the machine gunner at left, was the only survivor from this photographWilliam Rider-Rider
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Stretcher-bearers carry wounded Canadians towards an aid post during the Battle of PasschendaeleWilliam Rider-Rider
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November 1917: A Canadian soldier lights a German prisoner's cigarette at Passchendaele on the Western FrontReuters
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11 November 1917: Soldiers near the ramparts at YpresHulton Archive/Getty Images
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Aerial view of the village of Passchendaele before and after the Third Battle of Ypres, 1917IWM
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Circa 1925: A cemetery where troops killed in battle at Passchendaele are buriedGeneral Photographic Agency/Getty Images

The First World War, also known as the Great War, began on 28 July 1914 and lasted until 11 November 1918. It originated in Europe but became a global military conflict that killed more than 16 million people and changed the nature of warfare. On land, sea and in the air, 1914-18 was a war of new and experimental technology – technology that would increase casualty figures beyond the worst nightmares of previous conflicts.

Between 1914 and 1918, an estimated 400 million artillery rounds were fired in the narrow battlefield straddling France and Belgium.

World War One saw the introduction of many firsts in technological, scientific and societal innovations. Tanks were invented as a means of breaking the trench warfare stalemate. Chemical weapons in the form of deadly poison gases were used for the first time, leading quickly to the development of the first gas masks.