The 'Battle of the Lys' was fought between German and Allied forces during the German 'Gelb' invasion of Belgium and was the final major battle fought by Belgian troops before their surrender (24/28 May 1940).

The battle was the bloodiest of the 18-day campaign, and was was named after the Leie (Lys in French), the river at which the battle took place.

On 24 May, a heavy German attack forced Allied troops to fall back at Kortrijk over the Lys river onto the Belgian forces of Luitenant-generaal Walter Emile Coppens’s 1^st Division and Luitenant-generaal Gaston Lozet’s 3^rd Division. The Belgians had been persuaded to abandon the Scheldt river and withdraw in order to relieve British troops for an Allied counter-offensive, but strategically this achieved little to alleviate the situation at the front. With the Allied line facing four German divisions, Luitenant-generaal Richard Vander Hofstadt’s 9^th Division and Luitenant-generaal Jules Pire’s 10^th Division rushed in to reinforce the position. Luitenant-generaal Victor Michem’s II Corps launched a counterattack and captured 200 Germans. Belgian artillery fired effectively on the Germans, but the Allied lines were subject to numerous bombing raids and strafing runs, with negligible air support of their own. A German division from Menen moved up to Ypres, threatening divide the Belgian army and General the Lord Gort’s British Expeditionary Force. The Belgian 2^nd Cavalry Brigade and Luitenant-generaal Emile Janssens’s 6^th Division came in to support the area and managed to hold off the Germans.

On 25 May, the British, realising that further counter-offensives were no longer possible, began to withdraw to the port of Dunkirk for evacuation in 'Dynamo', and all hope of saving the Belgian army was lost. It became clear at this juncture that all the Belgians could achieve was to buy enough time for the Allies to complete their evacuation. The British spared a brigade and a machine gun battalion, their only reserves, to assist the Belgians in delaying the German advance. At 06.30, the 12^th Royal Lancers, an armoured car regiment, was despatched to the north of the Lys river to cover the left flank of Lieutenant General A. F. Brooke’s British II Corps and re-establish contact with the Belgians in the area. The regiment reported that the Belgians were retreating in the face of superior forces, and it was itself sporadically engaged with the Germans. In an order to his troops that day, King Leopold III informed his army that 'Whatever may happen, I shall share your fate.' Low morale prompted sections of the Belgian 5^th and 17^th Regiments to surrender the bridgehead at Meigem without a fight and in direct contravention of their officer’s orders, which were ignored. In one instance, demoralised soldiers shot their superiors. The elite Chasseurs Ardennais were deployed to the small village of Vinkt. Here the 1^st Division successfully repulsed numerous attacks by Generalleutnant Karl Kriebel’s 56^th Division of General Hermann Geyer’s IX Corps within Generaloberst Walter von Reichenau’s 6^th Army. Lieutenant Colonel G. Davy, head of the British military mission to the Belgian army’s headquarters, was told that the Belgians would be unable to extend their front any farther. Starting that night, 2,000 wagons were lined up side by side along the railway line between Roeselare and Ypres to act as an improvised anti-tank barrier.

By 26 May the Allied position was becoming desperate. The Belgians were struggling to hold Izegem, Nevele and Ronsele. The Chasseurs Ardennais held their ground against the 56^th Division, which was subsequently replaced by Generalleutnant Ernst Schaumburg’s 225^th Division. Generalleutnant Gerhard Kauffmann’s 256^th Division managed to cross over the canal at Balgerhoeck and attack Eeklo. The Belgian Lanciers Regiment abandoned Passchendaele and Zonnebeke, and British engineers blew up the Menin Gate bridge. Fresh German units threatened to split the Belgian and British lines, but their attack was blunted by a Belgian infantry division and a cavalry division. An additional infantry division maintained the integrity of the defensive line. By this time all of Belgium’s reserves had been committed, and auxiliary troops began arming themselves with 75-mm (2.95-in) guns from training centres to form the rear. The Belgian command now resorted to flooding the canals to contain the Germans. At 12.00 the Belgians informed the French head of the mission to their headquarters, Général de Corps d’Armée Pierre Louis Célestin Michel Champon, that 'the army has nearly reached the limits of its endurance'. At 18.00, Général de Corps d’Armée Georges Maurice Jean Blanchard, newly appointed commander of the French 1er Groupe d’Armées, arrived to inform Leopold that the British were withdrawing farther to the rear onto the line linking Lille and Ypres. Gort ordered Major General H. E. Franklyn to hold the dry Comines-Ypres Canal with his 5^th Division in order to cover the withdrawal toward Dunkirk. That evening, Leopold began making plans to relocate his headquarters to Middelkerke.

The Belgian army began to collapse on 27 May. The nation’s railways were out of service, the roads were clogged with 1.5 million refugees in addition to the 800,000 people already living in the area, ammunition and food were running short, and no fresh troops were available. The Belgians began destroying their artillery as they exhausted their munitions and retreated. By 11.00, the line had been breached to the north of Maldegem, in the centre near Ursel and to the right near Thielt and Roeselare. Bruges was the only major Belgian city not yet taken by the Germans. At 16.00, the Chasseurs Ardennais were forced to abandon Vinkt, which left the Germans in control. The chasseurs had lost 39 men while managing to kill 170 Germans. In the subsequent Vinkt massacre, 86 civilians in the village were killed by vengeful German troops. However, a counterattack by the 4^th Carabiners Cyclistes at Knesselaere resulted in the surrender of some 120 to 200 Germans.

Around the same time, the Belgian command came to accept the fact that '(1) From the national point of view, the Belgian army had carried out its task; it had resisted to the limit of its capacity; its units were unable to continue the fight. There could be no retreat to the Yser; it would do more to destroy the units than the fighting in progress; it would increase the congestion of the Allied forces to the highest pitch; and (2) from the international point of view, the dispatch of an envoy to ask for terms for the cessation of hostilities would have the advantage of allowing the Allies the night of the 27/28 May and part of the morning of 28 May, an interval that, if the fighting were continued, could be gained only at the cost of the complete destruction of the army.'

The Belgian army’s chief-of-staff, Luitenant-generaal Oscar Michiels, recommended that a representative be sent to the Germans to negotiate a ceasefire, and at 17.00 Leopold decided to send the army’s deputy chief-of-staff, Generaal-majoor Olivier Derousseaux, to the headquarters of General Georg von Küchler’s 18^th Army. Two French divisions were withdrawn by truck toward Dunkirk while Belgian flags and battle standards were hidden for safekeeping. A final order to retreat was issued from the Belgian army headquarters at 20.00. Derousseaux returned at 22.00 with the German reply that 'the Führer demands that arms be laid down unconditionally'. Leopold was disappointed by the demand but acknowledged that the Belgian army had no option but to comply, and at 23.00, with the full support of his staff, accepted the demand and agreed to a ceasefire at 04.00 on 28 May. Fighting continued on the line between Roeselare and Ypres until 06.00, when the troops operating there finally received the order to capitulate. Leopold made one final proclamation to his men: 'Plunged unexpectedly into a war of unparalleled violence, you have fought courageously to defend your homeland step-by-step. Exhausted by an uninterrupted struggle against an enemy very much superior in numbers and material, we have been forced to surrender. History will relate that the army did its duty to the full. Our honour is safe. This violent fighting, these sleepless nights, cannot have been in vain. I enjoin you not to be disheartened, but to bear yourselves with dignity. Let your attitude and your discipline continue to win you the esteem of the foreigner. I shall not leave you in our misfortune, and I shall watch over your future and that of your families. Tomorrow we will set to work with the firm intention of raising our country from its ruins.'

In spite of the Belgians' attempts to delay the Germans for as long as possible, the surrender angered the Allies, whose armies' north-western flank was now vulnerable to German attack. Moreover, French civilians became increasingly hostile to the Belgians in their midst.

The battle was one of the bloodiest of the 'Battle of Belgium'. Of Belgium’s 80,000 casualties during the German invasion, 40,000 occurred between 25 and 27 May.

Leopold’s decision to remain with his army and surrender was seen as traitorous by Hubert Pierlot and the Belgian government-in-exile, and after the war public suspicion of the king’s loyalties paved the way to the question royale dispute, which led to Leopold III’s July 1951 abdication of the throne in favour of his son, Baudouin.