On Saturday, South Sudan becomes a free and independent country. It is a well-deserved victory for its people. Under a 2005 American-backed political accord that ended two decades of civil war, the people of the mainly Christian territory voted overwhelmingly in January to secede from the Arab Muslim north. Still, celebrations in the capital, Juba, cannot obscure a sobering truth: building a functional new country will take decades of hard work. Responsibility falls primarily on South Sudan, but also on the United States and the international community that shepherded it. Africas 54th state is at the bottom of the developing world. Most people live on less than $1 a day. More than 10 percent of children do not reach the age of 5. Some 75 percent of adults cannot read. Meanwhile, festering disputes between north and south are stoking chaos in a land already bloodied by two million deaths in civil war. Sudan on Friday became the first state to recognize South Sudan. Sudans president, Omar Hassan al-Bashir, author of the murderous war in Darfur, said he would attend the festivities in Juba. But he also said he would continue the fight that erupted last month against forces loyal to the south in South Kordofan, an oil-rich region still under Khartoums control. Mr. Bashirs decision to order the United Nations to withdraw peacekeepers from South Kordofan is deeply worrisome. Major elements of the 2005 peace agreement are unresolved - such as which side will control the oil-rich region of Abyei, where fighting has also broken out; citizenship protections for minorities; where final borders will be set; how oil earnings will be shared (the south has 70 percent of the reserves). The two sides are dependent on each other. South Sudan needs the norths pipeline to get its oil to market. Sudan needs oil money to help pay its bills. Both need foreign investment and the north needs debt relief. They have a better chance of winning international support if they are at peace. As an incentive, the United States and its partners have offered to convene an international conference in September for South Sudan. That will allow South Sudans leaders to present their plans for encouraging desperately needed private investment. Washington gave Juba $300 million for education and housing and is promising more. International assistance should go forward only if South Sudan works constructively with Khartoum to bring stability to both countries. The Obama administration, correctly, is not taking Sudan off its terrorism list and normalizing relations until Khartoum fulfils the peace deal and ends the conflict in Darfur. China, Sudans main oil investor and arms supplier, should deliver a similar message to Bashir, who is under war crimes indictment, instead of receiving him with fanfare in Beijing and promising him new oil deals. The international community must persuade the two sides to avoid war and work to build a future for both Sudans. New York Times