British offensive (November–December 1917) on the Western Front during World War I that marked the first large-scale, effective use of tanks (tank) in warfare.

Carried out by the 3rd Army under General Sir Julian Byng in order to relieve pressure on the French front, the offensive consisted of an assault against the Germans' Hindenburg line along a 10-mile (16-kilometre) front some 8 miles (13 km) west of Cambrai in northern France. The chosen terrain, rolling chalk downland, was especially suitable for tank movement. Nineteen British divisions were assembled for the offensive, supported by tanks (476 in all, of which about 324 were fighting tanks; the rest were supply and service vehicles) and five horsed cavalry divisions. For the initial attack, eight British divisions were launched against three German divisions.

Attacking by complete surprise on November 20, the British tanks ripped through German defenses in depth and took some 7,500 prisoners at low cost in casualties. Bad weather intervened, however, so that the cavalry could not exploit the breakthrough, and adequate infantry reinforcements were not available. By November 29 the offensive had been halted after an advance of about 6 miles (10 km). On November 30 the Germans counterattacked with 20 divisions, and by December 5 the British had been driven back almost to their original positions. Casualties on both sides were about equal—45,000 each. Despite the British failure to exploit the initial success of their tanks, the battle demonstrated that armour was the key to a decision on the Western Front.