At the outbreak of World War II, Leopold tried to maintain Belgian neutrality, but after the German invasion in May 1940, he surrendered his country, earning him much hostility, both at home and abroad.

In 1944, they were moved to Germany and then Austria, before being liberated by the Americans, but banned for some years from returning to Belgium, where his brother Prince Charles, Count of Flanders, had been declared regent.

Leopold's first wife, Astrid of Sweden, died in a road accident while on a driving holiday in Switzerland in August 1935, being much mourned by the public.

In August 1914, when Belgium was invaded by Germany, King Albert allowed Leopold, then aged twelve, to enlist in the Belgian army as a private and fight in defence of the kingdom.

They originally intended to wait until the end of the war for the civil marriage, but as the new Princess of Réthy was soon expecting their first child, the ceremony took place on 6 December 1941.

They had three children in total: When World War II broke out in September 1939, the French and British governments immediately sought to persuade Belgium to join them.

On the first day of the offensive, the principal Belgian strong point of Fort Eben-Emael was overwhelmed by a daring paratroop operation and the defensive perimeter thus penetrated before any French or British troops could arrive.

After a short running battle that eventually involved the armies of all four belligerents, Belgium was overwhelmed by the numerically superior and better-prepared Germans.

Alan Brooke who commanded II Corps of the BEF thought that the 10th Belgian Division was in the wrong place and wanted to deploy north of Brussels to avoid "double-banking".

The French liaison officer, General Champon, told Brooke that Van Overstraeten had ascendancy over the king and had taken control, so it was useless to see the Chief of Staff.

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".

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.

A group of Belgian refugees in Paris placed a message at King Albert's statue denouncing his son as "your unworthy successor".

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.

Having since June 1940 desired a meeting with Adolf Hitler in respect of the situation of Belgian prisoners of war, Leopold III finally met with him on 19 November 1940.

Leopold wanted to persuade Hitler to release Belgian POWs, and issue a public statement about Belgium's future independence.

In refusing to publish a statement, Hitler preserved the king from being seen as cooperating with Germany, and thus engaged in treasonous acts, which would likely have obliged him to abdicate upon the liberation of Belgium.

[citation needed] Jozef-Ernest Cardinal van Roey, Archbishop of Mechelen, wrote an open letter to parish priests throughout the country announcing Leopold's second marriage on 7 December.

The letter from the Cardinal revealed that the king's new wife would be known as Princesse de Réthy, not Queen Lilian, and that any children they had would have no claim to the throne.

Winant, the American Ambassador to the Court of Saint James's, reported a Foreign Office official's concern regarding irredentist propaganda in Wallonia.

Because of the controversy about his conduct during the war, Leopold III and his wife and children were unable to return to Belgium and spent the next six years in exile at Pregny-Chambésy near Geneva, Switzerland.

[citation needed] Frans Henri van den Dungen, rector of the Free University of Brussels, wrote to Leopold on 25 June 1945 about concerns for serious disorder in Wallonia, "The question is not if the accusations against you are right or not [but that...] you are no longer a symbol of Belgian unity.

"[25] The president of the Belgian Chamber of Representatives, Frans Van Cauwelaert, was concerned that there would be a general strike in Wallonia and revolt in Liège.

The country stood on the brink of civil war, and Belgian banners were replaced by Walloon flags in Liège and other municipalities of Wallonia.

[27] To avoid tearing the country apart, and to preserve the monarchy, Leopold decided on 1 August 1950 to delegate his powers and duties to his 20-year-old son Baudouin, making the latter regent.

[citation needed] As of 2023, two of Leopold's grandsons are reigning monarchs: Grand Duke Henri of Luxembourg since 2000, and King Philippe of Belgium since 2013.