After nearly a week of false starts and confusion, South Sudanese opposition leader Riek Machar finally arrived at Ethiopia's Gambela airport on Saturday to fly to Juba to take up his position as vice president, theoretically ending the country's two-and-a-half year civil war. But a series of administrative disagreements with the rival Juba government of president Salva Kiir has prevented his journey, throwing the success of the long hoped-for peace process into doubt.