Battle of Loos - Wikipedia

On September 25, 1915, the Battle of Loos commenced. Chlorine gas deployed by the British was blown back into their own trenches. There were 59,000 British and 26,000 German casualties. From the article:

"The battle was the British part of the Third Battle of Artois, an Anglo-French offensive (known to the Germans as the Herbstschlacht (Autumn Battle). Field Marshal Sir John French and Haig (GOC First Army), regarded the ground south of La Bassée Canal, which was overlooked by German-held slag heaps and colliery towers, as unsuitable for an attack, particularly given the discovery in July that the Germans were building a second defensive position behind the front position. At the Frévent Conference on 27 July, Field Marshal French failed to persuade Ferdinand Foch that an attack further north offered greater prospects for success. The debate continued into August with Joffre siding with Foch and the commanders being over-ruled by Herbert Kitchener, the British Secretary of State for War, on 21 August.[1] On 3 May, the British decided upon use of poison gas in military operations in France. At a conference on 6 September, Haig announced to his subordinates that extensive use of chlorine gas might facilitate success despite the terrain, if the French and British were able to keep the attack secret and advance on a line towards Douai and Valenciennes...

French decided to keep a strong reserve consisting of the Cavalry Corps, the Indian Cavalry Corps and XI Corps (Lieutenant-General Richard Haking), which consisted of the Guards Division and the New Army 21st Division and 24th Division, which had recently arrived in France and a corps staff (some of whom had never worked together or served on a staff before). Archibald Murray, the Deputy Chief of the Imperial General Staff (DCIGS) advised French that as troops fresh from training, they were suited for the long marches of an exploitation rather than for trench warfare. French was doubtful that a breakthrough would be achieved. Haig and Foch, commander of the groupe des armées du nord (Northern Army Group), wanted the reserves closer, to exploit a breakthrough on the first day; French agreed to move them nearer to the front but still thought they should not be committed until the second day.[4]

Haig was hampered by the shortage of artillery ammunition, which meant the preliminary bombardment, essential for success in trench warfare, was insufficient. Prior to the British attack, about 140 long tons (140,000 kg) of chlorine gas was released with mixed success; in places the gas was blown back onto British trenches. Due to the inefficiency of contemporary gas masks, many soldiers removed them as they could not see through the fogged-up eyepieces or could barely breathe with them on, which led to some soldiers being affected by the British gas as it blew back. Wanting to be closer to the battle, French had moved to a forward command post at Lilliers, less than 20 miles (32 km) behind the First Army front. He left most of his staff behind at GHQ and had no direct telephone to the First Army, which attacked at 6:30 a.m. on 25 September, sending an officer by car to request the release of the reserves at 7:00 a.m."