The invasion of Belgium or Belgian campaign (10–28 May 1940), often referred to within Belgium as the 18 Days' Campaign, formed part of the greater 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 Belgium included the first tank battle of the war, the Battle of Hannut. It was the largest tank battle in history at the time but was later surpassed by the battles of the North African Campaign and the Eastern Front. The battle also included the Battle of Fort Eben-Emael, the first strategic airborne operation using paratroopers ever attempted.