The Second Battle of Ypres (22 April-25 May 1915) saw the Germans use chlorine gas for the first time in warfare. This attack caused devastating casualties among the Allied forces, and forced them into a temporary retreat. However, the Allies eventually rallied and managed to hold the line.
The Third Battle of Ypres (31 July-10 November 1917), also known as the Battle of Passchendaele, was one of the bloodiest and most costly battles of the entire war. The battle was fought between the Allied forces (primarily the British and Canadian troops) and the German Army over the control of the strategically important ridges of Passchendaele. The fighting was fierce and the casualties were enormous, with over 300,000 men killed or wounded on both sides.
In all, the battles of Ypres during World War 1 resulted in the death or wounding of over 500,000 men. The fighting at Ypres is often remembered as some of the most brutal and senseless of the entire war.