First World War.com - Battles - The Second Battle of Ypres, 1915

On May 25, 1915, the Second Battle of Ypres ended with 105,000 casualties. From the article:

"On 24 May a heavy German assault forced a further Allied withdrawal, although little extra ground was ceded. A want of supplies and manpower obliged the Germans to call off the offensive; all that they could do was to bombard the town. Even so, the German attacks had considerably reduced the size of the Allied salient. The highest ground had been lost and it was no more than three miles across and five miles deep.

Losses during the Second Battle of Ypres are estimated at 69,000 Allied troops (59,000 British, 10,000 French), against 35,000 German, the difference in numbers explained by the use of chlorine gas. The Germans' innovative use of gas set the trend for the rest of the war.

Although roundly condemned by the Allies as barbaric and reprehensible, sentiments echoed by many neutral nations, the Allies quickly developed their own form of gas warfare, with the British releasing gas canisters at Loos at the end of September 1915 (although the prevailing wind turned and wafted the gas back into the British trenches). All the allied countries had made extensive use of poison gas by the close of the war."