South Sudan, which is marking a decade of independence, is the world's youngest country and also one of the poorest, crippled by a devastating civil war that has left tens of thousands of people dead. A veteran rebel who slipped in and out of government, Vice President Riek Machar has been a central figure in South Sudan's bloody history, his charming gap-toothed smile masking a capacity for anger and ruthlessness. LONG PROFILE OF RIEK MACHAR