Women rest against a U.N. vehicle as they take cover from fighting on the perimeter of the U.N. base and compound in Juba, South Sudan, on July 8. (Eric Kanalstein/UNMISS via Associated Press)

THE WORLD’S youngest country is violently coming apart. South Sudan, which turned 5 on July 9, descended into fresh chaos last weekend as intense fighting erupted in the capital Juba. Between July 7 and Monday, more than 200 people were killed in clashes between forces loyal to President Salva Kiir and those loyal to his former deputy, Riek Machar. Some 3,000 people fleeing the violence have sought shelter at United Nations bases, which also have come under fire. South Sudan looks to be on the brink of yet another bloody civil war.

Despite signing a peace agreement in August, Mr. Kiir and Mr. Machar have squandered opportunities to put aside mutual distrust and de-escalate their two-year civil war. On Wednesday, Mr. Machar withdrew his troops from Juba; a cease-fire appears to be holding for now. Still, the events of the past several days are reminiscent of the run-up to the outbreak of the civil war in 2013. Recognizing the deteriorating situation, the United States is evacuating all non-essential staff from South Sudan and has deployed 47 troops to help protect the U.S. Embassy and personnel.

Africa cannot afford to see South Sudan descend into full-blown civil war. Since 2013, more than 2 million refugees have been internally displaced or flocked to neighboring countries already strained for resources. Thousands of refugees are expected to flow into Uganda in the next few days, though that country is already hosting some 235,000 South Sudanese. Kenya, too, is expected to receive an influx, despite Kenyan President Uhuru Kenyatta’s vows to close refugee camps that are home to nearly 600,000 refugees from various countries.

Even as South Sudan’s fragile peace has unraveled, discussions at the United Nations over imposing an arms embargo were postponed. On Monday, U.N. Secretary General Ban Ki-moon renewed his call for an immediate arms embargo; if enacted, it could increase the pressure on the warring parties and eventually reduce the bloodshed. The United States ought to publicly support the embargo and thereby show its willingness to take meaningful steps to prevent civilian deaths. The Obama administration should also support the strengthening of the U.N. peacekeeping force in South Sudan, as proposed by Mr. Ban and Mr. Machar. East African foreign ministers have supported a foreign military intervention, which may be necessary if the U.N. force’s mandate cannot be broadened.

No other country played a bigger role in the creation of South Sudan than the United States. The new nation was one of Hillary Clinton’s signature projects as secretary of state. Last August, President Obama helped pressure the two warring leaders to sign a peace agreement. Now is the time for Mr. Obama to speak up for the people of South Sudan and let them know that the United States will not stand by as the infant state drowns in its own blood.