The invasion of Belgium or Belgian campaign (10–28 May 1940), formed part of the larger 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 late summer and autumn of 1944, when it was liberated by the Western Allies.
Pre-battle plans
Belgium's strained alliances
The Belgian strategy for a defence against German aggression faced political as well as military problems. In terms of military strategy, the Belgians were unwilling to stake everything on a linear defence of the Belgian–German border, in an extension of the Maginot Line. Such a move would leave the Belgians vulnerable to a German assault in their rear, through an attack on the Netherlands. Such a strategy would also rely on the French to move quickly into Belgium and support the border garrisons there.
Belgium was wary of continuing its existing treaty alliance with France, the Franco-Belgian Accord of 1920 concluded on 7 September that year. Marshal Philippe Pétain had suggested to execute in case of war a French strike at Germany's Ruhr area using Belgium as a spring-board, in October 1930 and again in January 1933. Belgium feared it would be drawn into a war regardless, and sought to avoid that eventuality. The Belgians also feared being drawn into a war as a result of the French–Soviet pact of May 1935. The Franco-Belgian agreement stipulated Belgium was to mobilise if the Germans did, but what was not clear was whether Belgium would have to mobilise in the event of a German invasion of Poland.
The Belgians much preferred an alliance with the United Kingdom. The British had entered the First World War in response to the German violation of Belgian neutrality. The Belgian southern North Sea (often imprecisely referred to as "Channel") ports had offered the German Imperial Navy valuable bases, and their possible occupation would offer the German Kriegsmarine and the Luftwaffe bases to engage in strategic offensive operations against the United Kingdom in a coming conflict. The UK would thus seem to have a vested interest in protecting the area. However, the British government paid little attention to the concerns of the Belgians. This lack of commitment ensured the Belgian withdrawal from the Western Alliance, the day before the remilitarisation of the Rhineland. The lack of opposition to the remilitarisation served to convince the Belgians that France and Britain were unwilling to fight for their own strategic interests, let alone Belgium's. The Belgian General Staff was determined to fight for its own interests, alone if necessary.
The changing Belgian attitudes were also determined by fundamental internal social developments. The Flemish Movement became ever more powerful and was by its nature averse to French influences. In 1935, the socialist Paul-Henri Spaak came to power and had to appease a strong pacifist wing in his party. At the same time, the Walloons vehemently objected to any strategy abandoning most French-speaking areas to the Germans.
Belgian place in Allied strategy
The French government was infuriated at King Leopold III's open declaration of neutrality in October 1936. The French Army saw its strategic assumptions undermined; it could no longer expect cooperation from the Belgians in defending Belgium's eastern borders, which would allow it to stop a German attack well forward of the French border. The French were dependent on cooperation from the Belgians. Such a situation deprived the French of any prepared defences in Belgium to forestall an attack, a situation which the French had wanted to avoid as it meant engaging the German Panzer divisions in a mobile battle. The French considered invading Belgium immediately in response to a German attack on the country. The Belgians, recognising the danger posed by the Germans, secretly made their own defence policies, troop movement information, communications, fixed defence dispositions, intelligence and air reconnaissance arrangements available to the French military attaché in Brussels. Officially, the Belgian government retained a suspicious posture towards France, considering them as equal a danger to Belgian sovereignty as the Third Reich. French forces were not allowed to enter Belgium, even when the German plans to invade became imminent.
In 1940, the Allied plan to aid Belgium had become the Dyle Plan; the cream of the Allied forces, including French armoured divisions, would advance to the Dyle river in response to a German invasion. The earlier alternatives for an established Allied line had been either reinforcing the Belgians in the east of the country, at the already fortified Meuse–Albert Canal line, or holding the Scheldt river, thus linking the French border defences to the south with the Belgian forces protecting Ghent and Antwerp. However, the Albert Canal would probably take too much time to reach, while a Scheldt line would create a too long salient. Entrenching at the Dyle in central Belgium, the defences extending westwards along the wide and practically impassable Scheldt Estuary, therefore seemed to be the soundest defensive strategy.
The weakness of the Dyle plan was that it abandoned most of eastern Belgium to the Germans. This included the easily defensible Ardennes region which would have covered the northern flank of the Maginot Line, but also the modern fortifications at the Albert Canal and the Meuse. Militarily it would rotate the Allied right wing perpendicular to the French frontier defences east of Sedan, creating a vulnerable "hinge" point there. It also increased logistical problems for the British. It lengthened their supply lines with the Channel ports, while the connections with the stock piles in northern France would run parallel to their front and become easily cut by a slight German advance. Despite these risks inherent to committing forces to central Belgium and an advance to the Scheldt or Dyle lines, which would be vulnerable to an outflanking move from the south, Maurice Gamelin, the French commander, approved the plan and it basically remained the Allied strategy at the outbreak of war.
The British, with only a small army in the field and behind in rearmament, were in no position to challenge French strategy, which had become dominant in the Western Alliance. Having little ability to influence French decision-making, the British strategy for military action limited itself to planning a strategic bombing of the Ruhr industry.
Belgian military strategy
Following the official Belgian withdrawal from the Western Alliance, the Belgians refused to engage in any official staff meetings with French or British military staff for fear of compromising their neutrality. The Belgians did not regard a German invasion as inevitable and were determined that if an invasion did take place it would be effectively resisted by new fortifications such as Eben Emael.
The Belgians had taken measures to reconstruct their defences along their border with Germany upon Adolf Hitler's rise to power in January 1933. The Belgian government had watched with increasing alarm the German withdrawal from the League of Nations, its repudiation of the Treaty of Versailles and its violation of the Locarno Treaties. The government modernised fortifications at Namur and Liège, and established new lines of defence along the Zuid-Willemsvaart (the Maastricht–'s-Hertogenbosch canal), joining the Meuse, the Scheldt and the Albert Canal lines. Protection of the eastern frontier, based mainly on obstructing a number of roads by trees, landslides and destroying key bridges, was entrusted to new formations (frontier cyclist units and the newly formed Chasseurs Ardennais). By 1935, the Belgian defences were completed. Even so, it was felt that the defences were no longer adequate. A significant mobile reserve was needed to guard the rear areas, without which the protection against a surprise assault by German forces would not be sufficient. Significant manpower reserves would also be needed, but the public rejected a bill requiring longer military service and refresher training for fears that it would increase Belgium's military commitments, perhaps in conflicts far from home.
King Leopold III delivered a speech on 14 October 1936 before the Council of Ministers to persuade the people and the government that Belgium's defences needed strengthening. He stressed three main military arguments for Belgium's increased rearmament:
a) German rearmament and the complete re-militarisation of Italy and Russia had caused most other states, even pacifists like Switzerland and the Netherlands, to take exceptional precautions.
b) Vast changes in military methods, particularly in aviation and mechanization, meant that initial operations could now be of alarming force, speed and magnitude.
c) The lightning reoccupation of the Rhineland came with bases for the start of a possible German invasion moving close to the Belgian frontier.
On 24 April 1937, the French and British publicly declared that Belgium's security was paramount to the Western Allies and that they would defend their borders against aggression of any sort, whether directed solely at Belgium, or to obtain bases to wage war against "other states". The British and French released Belgium from her Locarno obligations to render mutual assistance in the event of German aggression toward Poland, while the British and French maintained their military obligations to Belgium.
Militarily, the Belgians considered the Wehrmacht stronger than the Allies, and that engaging in overtures to the Allies would make Belgium a battleground without adequate allies.
After the start of the Second World War in September 1939, the Belgians and French remained confused about what was expected of whom if or when hostilities actually commenced. The Belgians initially had been determined to hold the fortifications along the Albert Canal and the Meuse, without withdrawing, until the French Army arrived to support them. Gamelin, however, was not keen on pushing his Dyle plan that far. He was concerned that the Belgians would be driven out of their defences and would retreat to Antwerp, as in 1914. When it became clear that there was to be no French advance towards the Albert Canal, the Belgian divisions were planned to withdraw after resisting for only a short period and retreat westward to link up with the French/British forces. This information was not communicated to Gamelin, who assumed he had sufficient time to entrench at the Dyle.
As far as the Belgians were concerned, the Dyle Plan had advantages. Instead of the limited Allied advance to the Scheldt, or meeting the Germans on the Franco-Belgian border, the move to the Dyle river would reduce the Allied front in central Belgium by 70 kilometres (43 mi), freeing more forces for use as a strategic reserve. Belgium hoped to retain more Belgian territory, in particular the central industrial regions. It also had the potential to easily absorb Dutch and Belgian Army formations (including some twenty Belgian divisions). Gamelin justified the Dyle Plan after the defeat using these arguments.
On 10 January 1940, in an episode known as the Mechelen Incident, German Army major Hellmuth Reinberger crash-landed in a Messerschmitt Bf 108 near the Belgian village of Mechelen-aan-de-Maas. Reinberger was carrying initial plans for the German invasion of the Low Countries which, as Gamelin had expected, resembled the 1914 Schlieffen Plan in that they envisaged a German thrust through central Belgium (which was expanded by the Wehrmacht to include the Netherlands) and into France.