As recognition grows that the Battle of Passchendaele on October 12, 1917 was New Zealand's darkest day, there is a need to understand the full story of the offensive and why it was such a disaster.

World War I was a war of attrition and it became clear the loser would be the side that first ran out of equipment, ammunition and men. By early 1917 German submarines were sinking one out of every four merchant ships headed for Britain.

Admiral Jellicoe, the British First Sea Lord, warned that if nothing was done to stop this, Britain would not have enough supplies to go on fighting.

British Commander-in-Chief General Sir Douglas Haig's response was to attempt a breakthrough on the Western Front at Passchendaele. It proved a bloody, muddy disaster with the loss of 2412 New Zealanders' lives.

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Before the Passchendaele Offensive could be launched the Germans had to be removed from their positions on the Ypres Salient at Messines. A group of engineers tunnelled under the German lines and laid 23 high explosive mines.

At 3.10am on June 7, 19 of these mines were detonated simultaneously. It was the most powerful man-made explosion the world had seen. It was heard across the channel in London. It was so powerful it caused an earthquake.

Those Germans who were not killed abandoned their positions. The New Zealanders advanced and by 7am they had taken Messines and suffered relatively few casualties in what was generally regarded as one of the greatest military successes of the war.

After the success at Messines the way was open to make the advance on Passchendaele and a series of battles took place between July 31 and November 10 - Pilckem Ridge, Langemarck, Menin Road, Polygon Wood, Broodseinde, Poelcapelle and then Passchendaele itself.

During the Battle of Broodseinde on October 4 the New Zealanders gained a kilometre of territory and took Gravenstafel Spur, leaving only Bellevue Spur between them and the village of Passchendaele.

It was the spirit, determination, aggressiveness and courage of the New Zealanders that broke through the Germans' defence system and a bloody series of bayonet fights left the area littered with German dead.

All the German pillboxes were captured; an accomplishment that could only have been achieved by countless acts of individual bravery. They took a thousand prisoners and the kilometre gained in territory was a huge success in World War I terms.

Hundreds of New Zealanders, many not identified, are buried in Tyne Cot Cemetery, Passchendaele. Photo / File
Hundreds of New Zealanders, many not identified, are buried in Tyne Cot Cemetery, Passchendaele. Photo / File

New Zealand lost nearly 500 lives however, including Dave Gallaher, the captain of the 1905 Original All Blacks and Professor Herbert Milnes, the first dean of the Education Faculty at the University of Auckland.

However, less than a week after this victory came a disastrous defeat. The autumn of 1917 had been the wettest in Belgium for 70 years and the flat landscape around Passchendaele had been churned into a porridge of mud.

At Poelcapelle on October 9, several high ranking British officers wanted to halt the Passchendaele Offensive due to the deteriorating conditions but Field Marshall Haig would have none of that.

The New Zealand commander, Major-General Andrew Russell, tried to delay the battle saying, "The mud is a worse enemy than the Germans." But Haig was adamant.

The British artillery had pounded the German positions with 4.2 million shells in the two weeks before the battle and had completely destroyed the drainage system around Passchendaele.

The bombardments had been so destructive that they made the advance of troops impossible, yet at the same time they had not been able to take out the German defensive system of concrete bunkers on the high ground.

The battle terrain had been turned into a pitiless landscape with mud, rain-filled shell craters, war debris and uncut barbed wire entanglements sloping towards the German machine gun posts which stretched all the way along the Bellevue Spur leading to Passchendaele.

The mud meant the New Zealand artillery could not be properly positioned, so their barrages were weak and ineffective, some shells dropping short and causing deaths and injuries to our own soldiers.

Despite all that, the order was given to attack the Bellevue Spur before daybreak at 5.25am on October 12. When it began to rain with a vengeance, a human tragedy of epic proportions was inevitable and so began the most tragic day in New Zealand's history.

The New Zealanders advanced towards the ridge and as they tried to get through the uncut barbed wire, some of them up to their hips in mud, they were exposed to raking German machine gun fire from the front and the flanks.

Most were pinned down in the rain-filled shell craters and those who tried to get through the barbed wire were killed instantly.

By the end of the day 846 young New Zealanders had been killed. The total number of casualties - dead, wounded or missing - was 2700. It took two and a half days to clear the battlefield of bodies.

What was left of the New Zealand Division retreated and Passchendaele was eventually taken by Canadian forces on November 6 after two further battles. The village had been completely destroyed.

After five months of the Passchendaele campaign the allies had advanced just 8km and lost more than 300,000 soldiers.

Those lives however were all for nothing because in March 1918 the generals abandoned to the enemy every inch of territory gained to cover a new German offensive towards Ypres.

• Iain MacKenzie is president of the Passchendaele Society.