Nikki Haley, the U.S. ambassador to the United Nations, left, with Salva Kiir, the president of South Sudan. (Albert Gonzalez Farran/AFP/Getty Images)

SOUTH SUDAN never lived up to the soaring expectations heard at the time it became independent in 2011, but few would have thought that, nearly seven years later, it would be such a mess. On Feb. 2, the Trump administration imposed an arms embargo against South Sudan, “appalled by the continuing violence that has created one of Africa’s worst humanitarian crises.” The arms embargo will have little practical effect, since the United States does not sell any weapons to South Sudan, but it is better late than never, an important predicate to another drive at the United Nations Security Council for a global arms ban and more pressure on the nation’s murderous leadership.

This is not the happiest day for nation-building. The United States helped bring South Sudan to life. “After decades of conflict, the people of this region have reason to hope again,” Secretary of State Hillary Clinton declared on July 9, 2011, at the time of independence, promising the nation’s leaders “they can be assured that the United States will be a steadfast partner.” In recent years, the United States struggled to keep South Sudan from falling apart, but the bitter war between President Salva Kiir and his former deputy Riek Machar triggered a humanitarian catastrophe, including 1.96 million refugees abroad , 1.9 million displaced inside the country and widespread hunger. In the past three fiscal years, the United States has provided $2.9 billion in humanitarian funding, but when the U.S. arms embargo was announced, South Sudan’s first vice president, Taban Deng Gai, declared that Americans “are not our partners,” and the ambassador to Washington was recalled.

The Trump administration, led by Ambassador to the United Nations Nikki Haley, has shown welcome readiness to take a harder line with the errant leaders of South Sudan. Mr. Kiir’s perfidy — promising peace, waging war — destroyed the trust of many who had once believed in him. Repeated cease-fires and peace agreements have failed to stop armed clashes and brutality. Mr. Kiir’s disastrous rule means the United States must continue to search for other interlocutors in South Sudan. Although an arms embargo has previously not made it through the U.N. Security Council because of opposition from Russia, China and others, another attempt is called for. The United States also should exercise its clout with two allies, Ukraine and Uganda, to stop the flow of weapons to South Sudan.

The next steps in South Sudan must be calibrated carefully. The United States and other nations must pressure the leaders while trying to save the population from hunger and disease. No matter how frustrated with the continuing violence and lack of leadership, the United States cannot abandon the victims of war and famine. The United Nations recently estimated 7 million people in South Sudan will require some humanitarian aid this year. This fiscal year, the United States will provide $742 million. Nation-building is hard, and doubly hard in conditions such as this. The only way ahead is with clear-eyed realism and no illusions.