South Sudanese rebel leader Riek Machar was sworn in as first vice president on Tuesday, hours after he returned to the capital of Juba for the first time since conflict erupted more than two years ago.br Machar took up the post under the terms of a peace agreement reached eight months ago, implementation of which had been repeatedly delayed by disputes between Machar and the government of President Salva Kiir.br Macher's return is a crucial part of the peace agreement - it was Kiir's sacking of Machar as his deputy that ignited the two-year war in December 2013, which has killed thousands and displaced millions in the world's newest country.