The 2nd Australian Pioneer Battalion making a wagon track from planks of wood at Chateau Wood during the Battle of Passchendaele. Credit:Australian War Memorial It wasn't then called Passchendaele, the name of a rural village. Rather it was Third Ypres, as this ground had already been fought over in 1914 and again in 1915. Passchendaele opened on July 31, 1917 and finally ended on November 10. By that time, the five Australian divisions had withdrawn from the line, with 38,000 casualties, including 12,000 dead. For Australia, 1917 was by far the worst year of WWI. More than 21,000 dead comprised one-third of all Australians killed in the conflict. October 1917, with 6000 dead, was the worst single month. Yet this all started with such promise. With the Battle of the Somme called off at the end of 1916 and further progress in France unlikely, British commander-in-chief General Douglas Haig decided the big offensive for 1917 should be in Flanders, that area of Belgium adjacent to the border with France.

The Tyne Cot Cemetery in Passchendaele, Belgium pictured in an undated photo taken either during or after the Passchendaele battle. Credit:Australian War Memorial The aim was to capture high ground of the Gheluvelt Plateau, east of Ypres, although the term high is relative – no point is more than 50 metres higher than another. Haig's plan was to cut German supply lines and swing to the coast, and, in conjunction with an amphibious assault, seize ports from where German U-boats were raiding allied shipping. Those ports could then be used to land supplies for future operations. Poppies – each with a personalised message from a member of the British public – are currently on display at Tyne Cot Cemetery to mark the 100th anniversary of the Battle of Passchendaele in the fields near Ypres, Belgium. Credit:Getty Images Well before the battle proper, the allies undertook preparatory operations, with an attack on Messines Ridge on June 7, featuring the detonation of 19 huge mines in tunnels beneath German lines.

The movie Beneath Hill 60 tells of the role of Australian tunnellers in this multinational venture, which produced some of the then-biggest explosions in human history. German defences were devastated. Tourists visit World War One trenches in Ypres. Credit:Getty Images Haig's commander General Hubert Gough needed time to prepare and the offensive kicked off on July 31, with more than 250,000 British, French and Commonwealth soldiers attacking on a 13-kilometre front. Gough's "bite and hold" tactics proved effective. There was no thought of the great breakthrough – rather the idea was to seize a couple of kilometres of enemy territory, hold against inevitable counter-attacks, then repeat. Actors recreate the Battle of Passchendaeale for visitors. Credit:Getty Images

This required all objectives to be within range of massed allied artillery. After initial progress, the offensive stalled in August and Haig replaced Gough with General Hubert Plumer, who proposed step-by-step assaults with six-day pauses to allow artillery and supplies to be brought forward. The New Zealand memorial at the Tyne Cot Cemetery in Zonnebeke on the old Ypres Salient battlefields. Credit:Getty Images For the Anzacs, the first significant actions were Menin Road on September 20, followed by Polygon Wood on September 26 and Broodseinde on October 4. All were notable successes, though as always with significant casualties. But the night before Broodseinde, fine weather ended and what followed was the wettest autumn anyone could remember.

If that wasn't bad enough, months of shellfire had churned up the battlefield, destroying the drainage system which had kept this sodden area of northern Europe adequately dry for centuries. Australian War Memorial senior historian Ashley Ekins said as the front line advanced, mud made it progressively more difficult to bring forward the guns and the huge quantities of shells needed to obliterate German defences. Men and horses drowned in the quagmire. Evacuating a wounded soldier required multiple stretcher bearers and could take a day. Both sides had resorted to gas earlier in the war but for Passchendaele, the Germans employed a new horror: mustard gas. The attack on Poelcappelle on October 9 foundered in the mud, with 1250 Australian casualties for no gain. Then on October 12, Australian and New Zealand troops sought to seize Passchendaele, an attack which historian Chris Clark said had practically no chance of success.

In the mud, soldiers struggled just to remain on their feet, let alone keep pace with the advancing artillery barrage and assault German strong points. The result was 7000 dead and wounded, and the withdrawal of the battered Anzac force to recuperate. With 2700 dead, wounded and missing in a morning, October 12, 1917 remains New Zealand's darkest day. After five more costly attacks, Canadian troops finally took Passchendaele. With fighting bogged down and further objectives unattainable, Haig called off the offensive. He subsequently justified this as all about attrition – killing more Germans than they killed allies. The final toll remains disputed but current estimates suggest around a quarter million dead on each side. Official war correspondent Charles Bean wrote angrily at the time of what men were asked to do in these conditions.

"He was right. It was pushing soldiers beyond what they could possibly achieve," Ekins said. It's still asked why Haig insisted the battle continue. One reason is that high command never understood the conditions those at the front were confronting. British Prime Minister David Lloyd George, who had the ultimate authority, failed to act despite knowing the casualties. "They are all damned by the cost and futility of the campaign," he said. For the Australians, this was their lowest ebb. Units were depleted and exhausted, morale was low. Some thought the AIF could never again become an effective fighting force. Loading

Yet they did. With five divisions combined into a single corps commanded by General John Monash, the diggers would achieve a succession of victories in 1918 which materially contributed to the ultimate allied success. AAP