Despite being neutral at the start of World War II, Belgium and its colonial possessions found themselves at war after the country was invaded by German forces on 10 May 1940. After 18 days of fighting in which Belgian forces were pushed back into a small pocket in the north-west of the country, the Belgian military surrendered to the Germans, beginning an occupation that would endure until 1944. The surrender of 28 May was ordered by King Leopold III without the consultation of his government and sparked a political crisis after the war. Despite the capitulation, many Belgians managed to escape to the United Kingdom where they formed a government and army-in-exile on the Allied side.
The Belgian Congo remained loyal to the Belgian government in London and contributed significant material and human resources to the Allied cause. Many Belgians were involved in both armed and passive resistance to German forces, although some chose to collaborate with the German forces. Support from far right political factions and sections of the Belgian population allowed the German army to recruit two divisions of the Waffen-SS from Belgium and also facilitated the Nazi persecution of Belgian Jews in which nearly 25,000 were killed.
Most of the country was liberated by the Allies between September and October 1944, though areas in the far east of the country remained occupied until early 1945. In total, approximately 88,000 Belgians died during the conflict, a figure representing 1.05 percent of the country's pre-war population, and around 8 percent of the country's GDP was destroyed.
Background
During the 1930s, Belgium was still recovering from the destruction of World War I. Economically, Belgium was experiencing high unemployment in the aftermath of the Great Depression of 1929, and by 1932 unemployment stood at 23.5 percent though under the "New Deal-style" Plan de Man this had been reduced to around 15 percent by 1937.
The 1930s also saw the growth of several authoritarian and fascist political parties in both Wallonia and Flanders. In the 1936 elections, one of these, the Rexist party, gained 11.6 percent of the national vote. By 1939 however, extremist parties lost many of the seats that they had previously gained in new elections and political stability seemed to be returning.
Neutrality
As Belgium had suffered so much damage in World War I, there was little appetite within the country to involve itself in any potential European conflict. In October 1936, King Leopold III announced that Belgium would remain neutral in the event of another war in Europe as part of what he termed an Independent Policy (Politique d'Indépendance). To this end, the Belgian government tried to steer a path away from alliances: leaving the Locarno Treaty, repudiating a defence pact with France signed in 1920 and receiving a guarantee of neutrality from Nazi Germany in 1937.
During this period, the Belgian military was reorganized as an exclusively defensive force and began construction and modernization of fortifications around the country, particularly around Liège Province near the German border.
On the declaration of war between the United Kingdom, France, and Germany in September 1939, the Belgian government launched a crash re-armament program, augmenting the national defenses by creating the K-W Line linking the National Redoubt at Antwerp with the south along the River Dijle, just behind the main Fortified Position of Liège.
18 Days' Campaign
With the German invasion of Poland in September 1939, although still following a policy of neutrality, the Belgian government began general mobilization. By 1940, the army numbered between 600,000 and 650,000 men (nearly 20 percent of the male population of Belgium) making it approximately four times larger than the British Expeditionary Force and twice as large as the Dutch army at the time.
The invasion of Belgium by Nazi Germany started on May 10, 1940, under the codename Fall Gelb ("Case Yellow") as part of the wider invasion of France, the Netherlands and Luxembourg. The Belgian Albert Canal fortifications, some of the most modern defensive networks in Europe, proved almost useless. At Eben-Emael, the fort held by 1,200 Belgians was taken when the Germans deployed 500 glider-borne Fallschirmjäger against them, opening the border for Blitzkrieg-style warfare. Almost all of the air force's modern Hurricane fighters were also destroyed by the Luftwaffe on the ground at Schaffen airfield on May 10.