Surrender and constitutional crisis
On 24 May 1940, Leopold, having assumed command of the Belgian Army, met with his ministers for the final time. The ministers urged the king to leave the country with the government. Prime Minister Hubert Pierlot reminded him that capitulation was a decision for the Belgian government, not for the king, to make. The king indicated that he had decided to remain in Belgium with his troops, whatever the outcome. The ministers took this to mean that he would establish a new government under the direction of Hitler, potentially a treasonous act. Leopold thought that he might be seen as a deserter if he were to leave the country: "He was resolved to remain with his army and amidst his people so as to share their fate." Leopold had long had a difficult and contentious relationship with his ministers, acting independently of government influence whenever possible, and seeking to circumvent and even limit the ministers' powers, while expanding his own.
French, British, and Belgian troops were encircled by German forces at the Battle of Dunkirk. Leopold notified King George VI by telegram on 25 May 1940 that Belgian forces were being crushed, saying "assistance which we give to the Allies will come to an end if our army is surrounded". Two days later (27 May 1940), Leopold surrendered the Belgian forces to the Germans.
Prime Minister Pierlot spoke on French radio, saying that the king's decision to surrender went against the Belgian Constitution. The decision, he said, was not only a military decision but also a political decision, and the king had acted without his ministers' advice, and therefore contrary to the Constitution. Pierlot and his government believed this created an impossibilité de régner:
Should the king find himself unable to reign, the ministers, having observed this inability, immediately summon the Chambers. Regency and guardianship are to be provided by the united Chambers.
It was impossible, however, to summon the Belgian Chamber of Representatives or Belgian Senate at this time, or to appoint a regent. After the liberation of Belgium in September 1944, the government asked Leopold's brother, Prince Charles, to serve as regent.
After Leopold's surrender, the British press denounced him as "Traitor King" and "King Rat"; the Daily Mirror published a picture of Leopold with the headline "The Face That Every Woman Now Despises". A group of Belgian refugees in Paris placed a message at King Albert's statue denouncing his son as "your unworthy successor". French Prime Minister Paul Reynaud accused Leopold of treason. Flemish historians Valaers and Van Goethem wrote that Leopold III had become "The scapegoat of Reynaud", because Reynaud was probably already aware that the Battle of France was lost.
Leopold's surrender was also decried by Winston Churchill. In the House of Commons on 4 June 1940 he said:
At the last moment when Belgium was already invaded, King Leopold called upon us to come to his aid, and even at the last moment we came. He and his brave, efficient army, nearly half a million strong, guarded our left flank and thus kept open our only line of retreat to the sea. Suddenly, without prior consultation, with the least possible notice, without the advice of his ministers and upon his own personal act, he sent a plenipotentiary to the German Command, surrendered his army and exposed our whole flank and means of retreat.
In 1949, Churchill's comments about the events of May 1940 were published in Le Soir (12 February 1949). Leopold's former secretary sent a letter to Churchill saying that Churchill was wrong. Churchill sent a copy of this letter to the King's brother, Prince Charles, via his secretary André de Staercke. In his own letter Churchill wrote,
With regards to King Leopold, the words which I used at the time in the House of Commons are upon record and after careful consideration I do not see any reason to change them (...) it seemed to me and many others that the king should have been guided by the advice of his ministers and should not have favoured a course which identified the capitulation of the Belgian Army with the submission of the Belgian State to Herr Hitler and consequently taking them out of the war. Happily this evil was averted, and in the end, all came right. I need scarcely say that nothing I said at the time could be interpreted as a reflection upon the personal courage or honour of King Leopold.
De Staercke replied that Churchill was right: "The Prince, Monsieur Spaak [Belgian Foreign Minister Paul-Henri Spaak] and I read your text, which states the precise truth and seems perfect to us."
Belgian historian Francis Balace wrote that capitulation was inevitable because the Belgian Army was not able to fight any longer against the German army. Even Churchill admitted that their position was perilous. In a telegram to Field Marshal Lord Gort on 27 May, only one day before the Belgian capitulation, he wrote, "We are asking them to sacrifice themselves for us."
