Dyle Plan in the context of Grand Quartier Général (1939–1940)

The invasion of Belgium or Belgian campaign (10–28 May 1940), often referred to within Belgium as the 18 Days' Campaign (French: Campagne des 18 jours; Dutch: Achttiendaagse Veldtocht), formed part of the larger Battle of France, an offensive campaign by Germany during the Second World War. It took place over 18 days in May 1940 and ended with the German occupation of Belgium following the surrender of the Belgian Army.

On 10 May 1940, Germany invaded Luxembourg, the Netherlands, and Belgium under the operational plan Fall Gelb (Case Yellow). The Allied armies attempted to halt the German Army in Belgium, believing it to be the main German thrust. After the French had fully committed the best of the Allied armies to Belgium between 10 and 12 May, the Germans enacted the second phase of their operation, a break-through, or sickle cut, through the Ardennes, and advanced toward the English Channel. The German Army (Heer) reached the Channel after five days, encircling the Allied armies. The Germans gradually reduced the pocket of Allied forces, forcing them back to the sea. The Belgian Army surrendered on 28 May 1940, ending the battle.

The Battle of Sedan or Second Battle of Sedan (12–15 May 1940) took place in World War II during the Battle of France in 1940. It was part of the German Wehrmacht's operational plan codenamed Fall Gelb (Case Yellow) for an offensive through the hilly and forested Ardennes, to encircle the Allied armies in Belgium and north-eastern France. German Army Group A crossed the Meuse with the intention of capturing Sedan and pushing westwards towards the Channel coast, to trap the Allied forces that were advancing east into Belgium, as part of the Allied Dyle Plan.

Sedan is situated on the east bank of the Meuse. Its capture would give the Germans a base from which to take the Meuse bridges and cross the river. The German divisions could then advance across the open and undefended French countryside to the English Channel. On 12 May, Sedan was captured without resistance and the Germans defeated the French defences around Sedan on the west bank of the Meuse. German Luftwaffe bombing and low morale prevented the French defenders from destroying the bridgeheads. On 14 May, the British Royal Air Force (RAF) and the French Armée de l'Air jointly tried to destroy the bridgeheads, but the Luftwaffe prevented them from doing so. In large air battles, the Allies suffered high losses which depleted Allied bomber strength in the campaign.