Whereas the first and second battles of Ypres were launched by the Germans in 1914 and 1915 respectively, Third Ypres was intended as Sir Douglas Haig's Allied forces breakthrough in Flanders in 1917.
Haig had long mulled the idea of launching a major offensive in Flanders. It was his preferred choice for 1916, although in the event the Battle of the Somme took precedence that summer.
Meticulously planned, Third Ypres was launched on 31 July 1917 and continued until the fall of Passchendaele village on 6 November. The offensive resulted in gains for the Allies but was by no means the breakthrough Haig intended, and such gains as were made came at great cost in human terms.