The Battle of Loos took place from 25 September to 8 October 1915 in France on the Western Front, during the First World War. It was the biggest British attack of 1915, the first time that the British used poison gas and the first mass engagement of New Army units. The French and British tried to break through the German defences in Artois and Champagne and restore a war of movement. Despite improved methods, more ammunition and better equipment, the Franco-British attacks were largely contained by the Germans, except for local losses of ground. The British gas attack failed to neutralize the defenders and the artillery bombardment was too short to destroy the barbed wire or machine gun nests. German tactical defensive proficiency was still dramatically superior to the British offensive planning and doctrine, resulting in a British defeat.

The Western Front was one of the main theatres of war during the First World War. Following the outbreak of war in August 1914, the German Army opened the Western Front by invading Luxembourg and Belgium, then gaining military control of important industrial regions in France. The German advance was halted with the Battle of the Marne. Following the Race to the Sea, both sides dug in along a meandering line of fortified trenches, stretching from the North Sea to the Swiss frontier with France, the position of which changed little except during early 1917 and again in 1918.