The German armies involved were the Seventeenth Army under Otto von Below, the Second Army under Georg von der Marwitz and the Eighteenth Army under Oskar von Hutier, with a Corps (Gruppe Gayl) from the Seventh Army supporting Hutier's attack. Although the British had learned the approximate time and location of the offensive, the weight of the attack and of the preliminary bombardment was an unpleasant surprise. The Germans were also fortunate in that the morning of the attack was foggy, allowing the stormtroopers leading the attack to penetrate deep into the British positions undetected.
By the end of the first day the British had lost nearly 20,000 dead and 35,000 wounded, and the Germans had broken through at several points on the front of the British Fifth Army. After two days Fifth Army was in full retreat. As they fell back, many of the isolated "redoubts" were left to be surrounded and overwhelmed by the following German infantry. The right wing of Third Army became separated from the retreating Fifth Army, and also retreated to avoid being outflanked.