The First Battle of Passchendaele is actually a little misleading because it was just one battle within the larger Battle of Passchendaele campaign, which lasted from July 31 to November 6, 1917. (It was also called the Third Battle of Ypres. Yup. That's history for you.)
Either way, the First Battle took place around the village of Passchendaele in western Belgium during World War 1 on October 12, 1917, between the Allied (mostly British) forces and the German.
Soldiers struggle to move artillery guns in the thick mud.
This excursion begins what is a trend throughout much of this list in that miscommunication causes great harm. In this case, some Allied leaders believed that a major advance toward Passchendaele was taking place. Corrections were made too little too late. Bad weather conditions made it hard to navigate troops and almost impossible to relocate artillery. In fact, many guns were said to be left at their old positions and merely fired off at the border of their range.
This eventually led to significant losses for the Allied forces, totaling almost 13,000 dead, 845 of whom were said to have been killed or wounded in the muddy no man's land.
Over the course of all the battles involved in the Third Battle of Ypres, ~1.7 million Allied forces were said to have been involved while ~2.6 million German forces were. Each side was said to have lost no less than 200,000 troops, but possibly as many as 400,000. I’d like to see the rig that can run that number of troops in Ultimate Epic Battle Simulator.