The German occupation of Luxembourg in World War II began in May 1940 after the Grand Duchy of Luxembourg was invaded by Nazi Germany. Although Luxembourg was officially neutral, it was situated at a strategic point at the end of the French Maginot Line. On 10 May 1940, the German Wehrmacht invaded Luxembourg, Belgium and the Netherlands. Luxembourg was initially placed under a military administration, but later became a civilly administrated territory and finally was annexed directly into Germany. The Germans believed Luxembourg to be a Germanic state, and attempted to suppress what they perceived as alien French language and cultural influences. Although some Luxembourgers joined the resistance or collaborated with the Germans, both constituted a minority of the population. As German nationals, from 1942, many Luxembourgers were conscripted into the German military. Nearly 3,500 Luxembourgish Jews were murdered during the Holocaust. The liberation of the country by the Allies began in September 1944, but due to the Ardennes Offensive it was not completed until early 1945.

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 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.

The German official history stated that in the 18 days of bitter fighting, the Belgian Army were tough opponents, and spoke of the "extraordinary bravery" of its soldiers. The Belgian collapse forced the Allied withdrawal from continental Europe. The British Royal Navy subsequently evacuated Belgian ports during Operation Dynamo, allowing the British Expeditionary Force (BEF), along with many Belgian and French soldiers, to escape capture and continue military operations. France reached its own armistice with Germany in June 1940. Belgium was occupied by the Germans until the autumn of 1944, when it was liberated by the Western Allies.