A pretty field?

The picture shows a pleasant field in the Belgium countryside. On the 12th Oct 1917 about 1200 NZ soldiers were killed in 2 hours for almost no gain in ground. The worst day in NZ's history - worse than Gallipoli, worse than Erubus, worse than any day in WW2. How did this happen?? On the 4th Oct on dry ground with plenty of artillery support the NZ division made huge gains (2000 yards which was unheard of during 1917) towards Passendale (with the loss of 350 men which was considered light)...so the British Generals in their wisedom decided to send the NZ and Aussies in again 8 days later. This was despite nearly constant rain and almost no artillery support (guns were bogged down in the mud). The result - in the early morning of the 12th the NZers climbed over the top into uncut barbed wire and German machine guns. The losses were huge...two of the regiments who went in with 300-330 men each came out with less than 20. The Otago and Wellington regiments disappeared... Because the NZ Div hadn't been able to move forward, and the Australians had moved 500 meters, the Aussie left flank was exposed to German machine guns and they had to retreat. The Canadians finally took Passendale a few weeks later...the Canadian cemetry has the names of 1000's of men killed in persuit of this objective...