The Battle of the Lys, also known as the Fourth Battle of Ypres, was fought from 7 to 29 April 1918 and was part of the German spring offensive in Flanders during the First World War. It was originally planned by General Erich Ludendorff as Operation George but was reduced to Operation Georgette, with the objective of capturing Ypres, forcing the British forces back to the Channel ports and out of the war. In planning, execution and effects, Georgette was similar to (although smaller than) Operation Michael, earlier in the Spring Offensive.

The German attack zone was in Flanders, from about 10 kilometres (6.2 mi) east of Ypres in Belgium to 10 kilometres (6.2 mi) east of Béthune in France, about 40 kilometres (25 mi) south. The front line ran from north-north-east to south-south-west. The Lys River, running from south-west to north-east, crossed the front near Armentières in the middle of this zone. The front was held by the Belgian Army in the far north, by the British Second Army (under Plumer) in the north and centre and by the British First Army (under Horne) in the south.

The German attacking forces were the Sixth Army in the south (under Ferdinand von Quast), and the Fourth Army in the north (under Friedrich Sixt von Armin). Both armies included substantial numbers of the new stosstruppen, trained to lead attacks with the new stormtroop tactics.

The British First Army was a relatively weak force; it included several worn-out formations that had been posted to a "quiet sector". This included two divisions of the Portuguese Expeditionary Corps, which were undermanned, lacked almost half of their officers, had very low morale and were set to be replaced the day of the German attack.

The German plan was to break through the First Army, push the Second Army aside to the north, and drive west to the English Channel, cutting off British forces in France from their supply line which ran through the Channel ports of Calais, Dunkirk and Boulogne.

The German bombardment opened on the evening of 7 April, against the southern part of the Allied line between Armentières and Festubert. The barrage continued until dawn on 9 April. The Sixth Army then attacked with eight divisions. The German assault struck the Portuguese Second Division, which held a front of about 11 kilometres (6.8 mi). The Portuguese division was overrun and withdrew towards Estaires after hours of heavy fighting. The British 55th (West Lancashire) Division, to the south of the Portuguese in a more defensible position, pulled back its northern brigade and held its ground for the rest of the battle, despite attacks from two German reserve divisions. The British 40th Division (to the north of the Portuguese) collapsed under the German attack and fell back to the north.

Horne committed his reserves (First King Edward's Horse and the 11th Cyclist Battalion) to stem the German breakthrough but they too were defeated. The Germans broke through 15 kilometres (9.3 mi) of front and advanced up to 8 kilometres (5.0 mi), the most advanced probe reaching Estaires on the Lys. There they were finally halted by British reserve divisions. On 10 April, the Sixth Army tried to push west from Estaires but was contained for a day; pushing north against the flank of the Second Army, it took Armentières.

Also on 10 April, German Fourth Army attacked north of Armentières with four divisions, against the British 19th Division. The Second Army had sent its reserves south to the First Army and the Germans broke through, advancing up to three kilometres (2 mi) on a six-kilometre (4 mi) front, and capturing Messines.