According to UNESCO, the Union of the Peoples of Cameroon was banned by France in July 1955, went underground and waged an armed struggle from December 1956; its secretary-general Ruben Um Nyobè was killed by the French army on 13 September 1958. That same year, on 18 February, Ahmadou Ahidjo became Prime Minister and negotiated internal self-government and then independence with Paris: French trust territory Cameroon reached it on 1 January 1960, the green-red-yellow flag replacing the French flag at a ceremony in Yaoundé. A single party was formed on 1 September 1966; the Liberty Laws of 19 December 1990 again allowed parties other than the ruling one, and the first multiparty presidential election, on 11 October 1992, remains contested.