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 many chose to collaborate with the German forces. Support from 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 10 May 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 10 May.

The German breakthrough at Sedan, which had been thought impassable, meant that defenders of the K-W Line risked being outflanked, and had to withdraw on 16 May. The German invasion triggered a panic amongst Belgian civilians in the path of the advancing German army. By 11 May, the roads leading westwards, away from the fighting, were blocked by refugees, hampering the eastward advance of French and British forces. It is estimated that around two million civilians fled their homes during the campaign.

The government's policy of neutrality had left Belgium with an outdated and ill-equipped army and air force. Above all, the army possessed only 16 battle tanks16 tanks of the French Renault ACG-1 type were in service in 1940, in addition to 270 other armoured vehicles, mostly of the lightly armoured T-13 and T-15 types. between its two cavalry divisions for political reasons as they had been considered too "aggressive" for the army of a neutral power. The air force, hurriedly reorganised in May 1940, was taken by surprise and could only field 180 serviceable aircraft out of its total of 234.

The military held out against German forces for 18 days, against overwhelming odds. On 28 May, forced into a small pocket along the Leie river and after failed attempts to broker a ceasefire on the 27th, the Belgian king and military surrendered unconditionally. Belgian casualties during the campaign numbered some 6,000 killed and 15,850 wounded. Some 112,500 French and Belgian troops escaped to the UK via Dunkirk but the majority of the Belgian survivors were made prisoners of war and many were not released until the end of the war.

With the surrender of the Belgian army, the government, led by Hubert Pierlot, fled first to Paris and formed a government in exile in Bordeaux. After the Fall of France, the government transferred to Eaton Square, London.

Surrender of Leopold III

Leopold III, king and commander in chief of the Belgian army, surrendered personally to German forces on 28 May, contrary to the advice of Pierlot's government, having personally decided that the Allied cause was lost. His decision was fiercely criticized by the French prime minister, Paul Reynaud and by Pierlot in a radio broadcast on 28 June 1940, where he declared Leopold's decision to be "an event without precedent in history".

The king remained in Belgium during the war as a German prisoner while the government went into exile and continued military action in the Allied cause. Unlike the Netherlands and Luxembourg where the monarchy was repressed or had joined the government in exile, Leopold III remained prominent in the occupied territory, and coins and stamps produced during the occupation continued to carry his face or monogram. Nevertheless, Leopold remained a focus for resistance, his position explained by the slogan "Belgium is captive! Long live Belgium! The King is captive! Long live the King!" While imprisoned, he sent a letter to Adolf Hitler in 1942 which has been credited with saving an estimated 500,000 Belgian women and children from forced deportation to munitions factories in Germany. In November 1940, Leopold visited Hitler in Berchtesgaden where he asked for Belgian prisoners of war to be freed.

After the war, allegations that Leopold's surrender had been an act of collaboration provoked a political crisis, known as the Royal Question, about whether he could return to the throne, which ultimately ended with his abdication.

German occupation

Life in occupied Belgium

Belgium was run by a German military government under General Alexander von Falkenhausen and Eggert Reeder until July 1944, and then by Reichskommissar Josef Grohé until liberation. The German government levied the costs of the military occupation on the Belgians through taxes, while also demanding "external occupation costs" (or "Anti-Bolshevik charge") to support operations elsewhere. In total, Belgium was forced to pay nearly two-thirds of its national income for these charges, a figure equaling 5.7 billion Reichsmarks.

As in all occupied countries in Europe, food, fuel and clothing were strictly rationed by the German authorities. Even with the stringent rationing, the food and materials which civilians should officially have been entitled to were not always available. A significant black market also existed in the country, supplying food illegally at very high prices to those that could afford it. Information and the press were strictly controlled by the German government and news was greatly restricted. Nevertheless, the sales of collaborationist newspapers like Le Soir and the newspapers of pro-collaborationist political parties like Le Pays Réel remained high. A large number of underground newspapers were also published and distributed – the underground paper La Libre Belgique achieved a circulation of 30,000.

Occupied Belgium was also targeted by the Allied bombers from both the British RAF and American USAAF. The policy led to high civilian casualties as bombs missed their intended targets and fell on civilian areas. In a raid on the Erla Motor Works in the Belgian town of Mortsel (near Antwerp) in April 1943, just two bombs dropped by the B-17s of the U.S. 8th Air Force fell on the intended target. The remaining 24 tons of bombs fell on civilian areas of the town, killing 936 and injuring 1,600 more in just eight minutes. The Allied policy was condemned by many leading figures within Belgium, including Cardinal van Roey.

Around 375,000 Belgians also served in labour programs within Germany during the war, working in manual jobs in industry or agriculture for the German war effort. Though nearly 180,000 Belgians signed up before conscription began in 1941, most were conscripted after that date and worked as forced labour against their will.

200,000 Belgian military prisoners of war, who had been captured in 1940, were also transported to Germany. Most were used as forced labour and paid only a nominal sum. About 80,000 (mainly Flemish) prisoners were returned to Belgium between late 1940 and 1941, but many remained in captivity until the end of the war. They were often kept in very poor conditions and around 2,000 died.

Collaboration

During the period of Nazi occupation, some Belgians collaborated with their occupiers. There were pro-Nazi political organizations in both Flemish and Walloon communities before and during the war. The most significant were DeVlag, Verdinaso and Vlaams Nationaal Verbond (VNV) in Flanders as well as the Catholic Rex movement in Wallonia. Each of these movements had subtly different ideologies, their own paramilitary forces and printed their own newspapers. These organisations were also instrumental in encouraging Belgians to enlist into the German Army. Unlike the German-style National Socialist agenda of DeVlag, VNV appealed directly to a Flemish separatist agenda, though this message was never the main source of their popularity. Infighting between the groups, particularly VNV and DeVlag, was considerable.