JUBA, South Sudan — The rebel leader Riek Machar returned to the capital, Juba, to celebrate a peace deal on Wednesday, more than two years after fierce fighting erupted in the city and forced him to flee the country.To further reinforce the peace deal, President Salva Kiir ordered the release of a jailed adviser to Mr. Machar, along with a rebel spokesman.Mr. Machar flew in with his wife and a small retinue. He met with Mr. Kiir, his long-time rival, and regional leaders who have pressed the two men to end the civil war.“We need peace,” Mr. Machar told thousands of people who gathered at a ceremony marking the peace accord that he, Mr. Kiir and the leaders of other groups signed last month.South Sudan, the world’s youngest nation, plunged into conflict in 2013 after Mr. Kiir sacked Mr. Machar as vice president.Troops loyal to both men clashed in the capital that December and ethnically charged fighting soon spread, shutting down oil fields, forcing millions to flee and killing hundreds of thousands of people.There has been sporadic fighting since the latest agreement was signed, but violence has died down.Mr. Kiir hosted Wednesday’s ceremony at the same spot where South Sudan’s leaders celebrated independence from Sudan in 2011 after decades of war fueled by oil, ethnicity and religion.Mr. Machar fled to the neighboring Democratic Republic of Congo in 2016 after fighting broke out again in the capital, wrecking an earlier peace deal and killing hundreds. He later traveled to South Africa, where he was held under house arrest until this year.