Battle of the Lys ( 1918 ) The Battle of the Lys ( 7 – 29 April 1918 ) also known as the Lys Offensive , the Fourth Battle of Ypres , the Third Battle of Flanders , and , was part of the 1918 German offensive in Flanders during World War I , also known as the Spring Offensive . It was originally planned by General Ludendorff as Operation " George " but was reduced to become 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 . Background . Strategic developments . The German attack zone was in Flanders , from about east of Ypres in Belgium to east of Béthune in France , about south . The front line ran from NNE to SSW . The Lys River , running from southwest to northeast , 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 . Prelude . Tactical developments . The German attacking forces were the Sixth Army in the south ( under von Quast ) , and the Fourth Army in the north ( under Sixt von Armin ) . Both armies included substantial numbers of the new " Stosstruppen " , trained to lead attacks in the new Hutier 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 . German plan of attack . The German plan was to smash through First Army , push 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 . Battle . Battle of Estaires ( 9 – 11 April ) . 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 2nd Division , which held a front of about . The Portuguese division was overrun and withdrew towards Estaires . The British 55th 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 ( 1st King Edward ' s Horse , 11th Cyclist Battalion ) to stem the German breakthrough but they too were defeated . The Germans broke through of front and advanced up to , the most advanced probe reaching Estaires on the Lys . There they were finally halted by British reserve divisions . On 10 April , Sixth Army tried to push west from Estaires but was contained for a day ; pushing north against the flank of Second Army , it took Armentières . Battle of Messines ( 10 – 11 April ) . Also on 10 April , German Fourth Army attacked north of Armentières with four divisions , hitting the British 19th Division . Second Army had sent its reserves south to aid First Army , and the Germans broke through , advancing up to on a front , and capturing Messines . The 25th Division to the south , flanked on both sides , withdrew about . By 11 April , the British situation was desperate ; it was on this day that Haig issued his famous " Backs to the wall " order . Battle of Hazebrouck ( 12 – 15 April ) . On 12 April , Sixth Army renewed its attack in the south , towards the important supply centre of Hazebrouck , another to the west . The Germans advanced some and captured Merville , Nord . On 13 April they were stopped by the Australian 1st Division , which had been transferred to the area . The British 5th and 33rd Divisions were also moved there . Battle of Bailleul ( 13 – 15 April ) . Between 13 and 15 April , the Germans drove forward in the centre , taking Bailleul , west of Armentières , despite increasing British resistance . Plumer assessed the heavy losses of Second Army and the defeat of his southern flank and ordered his northern flank to withdraw from Passchendaele to Ypres and the Yser Canal ; the Belgian Army to the north conformed . First Battle of Kemmel ( 17 – 19 April ) . The Kemmelberg is a height commanding the area between Armentières and Ypres . On 17 – 19 April , Fourth Army attacked there but was repulsed by the British . Battle of Béthune ( 18 April ) . On 18 April , Sixth Army attacked south from the breakthrough area toward Béthune but was repulsed . Second Battle of Kemmel ( 25 – 26 April ) . French Marshal Foch had recently assumed supreme command of the Allied forces and on 14 April agreed to send French reserves to the Lys sector . A French division relieved the British defenders of the Kemmelberg . On 25 and 26 April , Fourth Army made a sudden attack on the Kemmelberg with three divisions and captured it . This success gained some ground but it made no progress toward a new break in the Allied line . Battle of the Scherpenberg ( 29 April ) . On 29 April , a final German attack captured the Scherpenberg , a hill to the northwest of the Kemmelberg . Aftermath . Analysis . More French reinforcements arrived in the latter part of April , the Germans had suffered heavy casualties , especially among the and attacks toward Hazebrouck failed . It was clear that " Georgette " could not achieve its objectives ; on 29 April the German High Command called off the offensive . Casualties . In 1937 the British Official Historians gave casualties from as and a similar number of German casualties . Total casualties since 21 March were British : and German : In 1978 Middlebrook wrote of casualties , and Middlebrook estimated French casualties as German as wounded . In 2002 Marix Evans recorded casualties and the loss of British losses of and and French losses of and twelve guns . In 2006 Zabecki gave and casualties .