A British tank crossing a trench in maneuvers.
November 20 1917, Cambrai–The British had been using tanks in battle for over a year. While they had occasionally been quite effective on a tactical level, they had not yet produced the breakthrough that had been hoped for. The staff of General Byng’s Third Army had devised a plan for a surprise attack with massed tanks around Cambrai that hoped to do just that, which Haig approved in mid-October.
The plan called for over three hundred tanks, to be moved up to the front the day before so as to not alert the Germans. There would be no preliminary bombardment, not even a registering of artillery ranges–the artillery would be calibrated electrically beforehand. The ground was dry around Cambrai (unlike at Ypres), and the German troops in the area were not first-rate–Cambrai was occasionally nicknamed the “Flanders Sanitarium,” a place for recuperation from the fighting around Ypres. The Germans had some intelligence reports of an impending attack, but the complete lack of accompanying typical British preparatory actions meant the attack would come as a complete surprise.
At 6:20 AM, a huge rolling barrage erupted on a six-mile front with no warning but the sound of the tanks’ engines. The tanks followed close behind, spewing machine gun fire as they went. The infantry advanced behind the tanks, crossing the enemy trenches on bridges dropped by the tanks, supporting them and clearing out any pockets of resistance that remained. On the left and right of the British attack, the British advanced up to five miles, in some places breaking through the entirety of the Hindenburg Line; the way to Cambrai was open. However, there were not enough reserves to exploit the breakthrough immediately, and the cavalry had difficulty making it through the barbed wire and other wreckage of the battlefield.
The British were also stymied by extreme delays in the center, around Flesquières, where the British commander kept his infantry 150-200 yards behind the tanks, believing that this would keep them protected from German artillery fire that was sure to fall on the tanks. This meant that the tanks were entirely unsupported by infantry as they went over a ridge, and the Germans were able to concentrate fire on the tanks and knock out eleven of them. The local German commander had been especially worried about tanks and had trained his artillery to fire on moving targets. One artillery sergeant was able to take out five tanks before the infantry caught up with him; in other cases the tanks were taken out by machine guns with armor-piercing rounds or even individual soldiers with cluster grenades. The resulting delay meant that the British were not able to break through in the center as they had elsewhere.
Nevertheless, in a single day the British had advanced further than they had in several months at Ypres, with perhaps 2% of the casualties. Church bells rang in Britain for a victory, the first time since the start of the war.
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Sources include: John Keegan, The First World War; Robert B. Asprey, The German High Command at War.