On July 31, 1917 the Battle of Passchendaele began. It slogged on for 3 months, 1 week and 3 days, causing staggering losses on both sides. 

Battle of Passchendaele + Post Card
Part of the The Great War Centenary series
Issued by Belgium Post 2017
Designer: Kris Demey

By the end of the battle, the Allied forces suffered 275,000 casualties, including 38,000 Australians, 5,300 New Zealanders, and more than 15,600 Canadians. The German losses tallied to 220,000 killed or wounded.