Leopold III, French in full Léopold-Philippe-Charles-Albert-Meinrad-Hubertus-Marie-Miguel, Dutch in full Leopold Filips Karel Albert Meinrad Hubertus Maria Miguel, (born November 3, 1901, Brussels, Belgium—died September 25, 1983, Brussels), king of the Belgians, whose actions as commander in chief of the Belgian army during the German conquest of Belgium (1940) in World War II aroused opposition to his rule, eventually leading to his abdication in 1951.

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The son of Albert I and his consort Elisabeth of Bavaria, Leopold served as a private soldier during the final campaign of World War I. On November 10, 1926, he married Princess Astrid of Sweden (died August 1935); their children were Joséphine-Charlotte, Baudouin, and Albert. Leopold became king of the Belgians following his father’s death on February 17, 1934. Favouring an independent foreign policy, but not strict neutrality, he withdrew Belgium from its defensive alliance with France and from the Pact of Locarno—a peace agreement among Germany, France, Belgium, Italy, and Great Britain—after German occupation of the Rhineland (1936). Determined to resist aggression with help from Britain and France, he sponsored construction of a fortified defense line from Antwerp to Namur, facing Germany.

After the outbreak of World War II, Leopold assumed supreme command of the Belgian army. In May 1940, as the Allies undertook the evacuation of hundreds of thousands of troops from the French seaport of Dunkirk, Belgian forces at the Leie (Lys) River in Belgium battled the advancing Germans. Leopold was forced to surrender his encircled forces on May 28. The Belgian government’s repudiation of his decision to remain with his troops, rather than join the London government in exile, laid the foundation for the postwar conflict over his claim to the throne. Leopold was held prisoner by the Germans at his royal château near Brussels until 1944 and then in Austria to the end of the war. His letter to Adolf Hitler in 1942 is credited with saving an estimated 500,000 Belgian women and children from deportation to munitions factories in Germany. He married Mary-Lilian Baels on September 11, 1941, whom he made princesse de Réthy; their children were Alexandre, Marie-Christine, and Maria Esmeralda.

After his brother Charles was appointed regent in 1944, Leopold remained in Switzerland (1945–50), awaiting resolution of the “royal question,” the controversy over his pending return to the throne. In a plebiscite held March 12, 1950, nearly 58 percent of the voters favoured the king’s return, largely reflecting Catholic Fleming support. But unrest fomented by Liberal, Socialist, and Walloon opposition led Leopold to renounce his sovereignty on August 11, 1950, in favour of his son Baudouin, who became king the following year. Leopold and the princesse de Réthy continued to live in Laeken, however, the traditional home of Belgian kings, until his son’s marriage in 1960. Critics of Leopold felt that his residence in Laeken gave him too much influence over King Baudouin.

King Leopold III with his son Baudouin. Encyclopædia Britannica, Inc.