 When the Mauritanian miners went on a two-month strike in the late 1960s the army intervened and eight miners were killed.Left-wing opposition to the government mounted and some miners formed a clandestine Marxist union in 1973.President Ould Daddah survived the challenge from left-wing opponents by nationalising the company in 1974 and withdrawing from the franc zone, substituting the ouguiya for the CFA.