Loos is situated in France, it is South West of Lens, also in France, and is close to the Belguim border.
The Battle of Loos was an offensive carried out by British and French forces against the Germans in September 1915, it's intention was to recover the mining district around the towns of Loos and Lens and additionally, if possible, to capture Lille. At the same time it was hoped that the action would divert German troops from the Champagne offensive that was being carried out by French forces.
The German positions were well fortified with a large number of steel and concrete pillboxes and shelters. The British forces included a number of battalions from Kitchener's "New Army" which was being used for the first time, it was also the first time British forces used gas. Unfortunately the wind changed direction and the gas was blown back into the British lines but the cylinders were quickly shut off and little damage was done. The attack managed to drive a salient some way into the German lines but British reserves were badly organised and before they could be brought up a German counter-attack drove the British back to their starting positions. A second attack south of the Hohernzollern redoubt managed to gain some 3,500 yards and this was retained. About 250,000 British and a similar number of French forces were engaged in the battle. British losses were about 60,000 including three generals, French casualties are not known but are probably higher than the British.