German forces again invaded the country in May 1940, and 40,690 Belgians, over half of them Jews, were killed during the subsequent occupation and The Holocaust. From September 1944 to February 1945 the Allies liberated Belgium. After World War II, a general strike forced King Leopold III to abdicate in 1951 in favour of his son, Prince Baudouin, since many Belgians felt he had collaborated with Germany during the war. The Belgian Congo gained independence in 1960 during the Congo Crisis; Ruanda-Urundi followed with its independence two years later. Belgium joined NATO as a founding member and formed the Benelux group of nations with the Netherlands and Luxembourg.