Two previous battles had taken place upon the Ypres Salient: the First Battle of Ypres (October-November 1914) and the Second Battle of Ypres (April-May 1915) as Haig wanted to reach the Belgian coast to destroy the German submarine bases there.
The British were further encouraged by the success of the attack on Messines Ridge on 7 June 1917, when nineteen huge mines were exploded simultaneously beneath German lines.
The Battle of Passchendaele (officially Third Ypres) began on 31 July, but the constant shelling had destroyed drainage systems and within a few days, the heaviest rain for 30 years had turned the battlefield into a quagmire, eventually becoming so bad that men and horses could drown in it.
On 16 August the attack was resumed, but stalemate reigned until the weather improved, prompting another further attacks, including the Battle of Menin Road Ridge, the Battle of Polygon Wood and the Battle of Broodseinde. Passchendaele village was eventually captured by British and Canadian forces on 6 November. The Allies lost 325,000 casualties in this battle, while the Germans lost 260,000.