The Kingdom of Belgium didn't trust Adolf Nazi and thus started to mobilise soon after the start of World War II, especially after the Mechelen affair. When the Wehrmacht invaded without declaration of war on the 10th of May 1940, more than 600.000 men, lead by king Leopold III, were ready. They did lack tanks and anti-air, though.
The advancement of the Wehrmacht during mid-May
The Wehrmacht advanced quickly. On the 11th of May, the fortress of Eben Emael (manned by 650, taken by 85 paratroopers!) and the fortifications along the Albert Canal fell. The Dutch army couldn't assist, because it was cut off, and had to capitulate on May 14th. At the same time, the biggest tank battle of history at Hannut ended - with the Germans having higher losses than the French! - without people taking much notice. On the 16th of May, the Wehrmacht managed a breakthrough at Sedan, which had been considered impossible, so the Allies retreated behind Scheldt/Escaut river. Most of Belgium was already occupied. Panic resulted, two Million Belgians were fleeing, streets were full of them, which impedimented the movement of Allied soldiers. Until the 20th of May, Wehrmacht troops under Kleist arrived at the Channel; the remains of the Belgian Army, the British Expedition Corps under Lord Gort, and three French armies were cut off from France. On May 24th, the Wehrmacht took Boulogne and surrounded Calais, which worsened Allied chances for evacuation. General Sir William Edmund Ironside, Chief of the Imperial General Staff, commented: "At the moment, it looks like the greatest military disaster in history." After the Wehrmacht had taken Dunkirk on the 25th, king Leopold decided to capitulate - without asking his allies - on the 27th, which his army started doing on the same day. The country had left the war now.
Under German occupation[]
The country was put under Military administration, with General Alexander von Falkenhausen in charge. There was disunion among the Nazis how to deal with the bilingual country, especially the French-speaking Walloons and the capital of Brussels. There were collaborators like the Flemish VNV and the Rexist Party under Léon Degrelle, but those had just a small part of the population behind them. The communities of Eupen, Malmedy and Moresnet which had been ceded by the Reich after World War I returned to Germany.