IN THE MARSHLAND OF SOUTH SUDAN — BAREFOOT and shellshocked, the survivors trickle into a village here with unimaginable stories of rape, castration and mass murder committed by a government that the United States helped install.

This civil war here in South Sudan will be a top item on President Obama’s agenda during his visit to Africa this month, and I wish he could talk to these survivors.

Gatkuoth Kueah Yak tells me he watched from a distance as South Sudan government soldiers tied up his 15 children and put them in a grass hut. And then, he says, he watched as the soldiers torched the hut and burned his family alive.

Gatwech Them Manuar says he saw three young boys, aged 3 to 7, who had been castrated by government soldiers and left to bleed to death. He says he also saw two infants who were killed by soldiers bludgeoning them against a tree.

Nyakong Riek tells me that when government soldiers attacked her village, she ran with her 2-year-old son. “I was trying to pull him along,” she said. “But bullets were flying, and I couldn’t pull him fast enough. So I left him.” Dazed and sleepless, she says wistfully that she just hopes that the boy died quickly.

These survivors are of Nuer ethnicity, and they say the South Sudan Army — disproportionately composed of the rival Dinka tribe — targeted them for that reason. Indeed, they say some Dinka soldiers mockingly cut the faces of Nuer women they raped to replicate the kind of decorative scarring used by Nuer men.

I interviewed survivors on a grassy island, surrounded by rivers and marshland, where they have taken refuge (I reached the area by helicopter with a team from the United Nations World Food Program). The predations the Nuer describe happened in May in Unity State, and they say thousands of other survivors are stuck in the marshes, preyed upon by crocodiles, still trying to reach safety.

I shared the world’s exhilaration in 2011 as South Sudan celebrated independence from Sudan and became the world’s newest nation. But now it’s difficult not to feel despair.

South Sudan is rived by civil war and collapsing economically; it may be on a trajectory to a failed state with far-reaching consequences for the region. The country suffers worsening famine and mind-boggling corruption. It is led by a president, Salva Kiir, whom President George W. Bush and President Obama both tried to nurture. I’ve known Kiir for a decade and he surely faced huge challenges — but he and other leaders have failed his country.

South Sudan’s civil war erupted 18 months ago between Kiir’s army and the forces of the vice president, Riek Machar. Sudan has armed Machar, and all factions in the war have behaved brutally. The recent slaughter by government soldiers may be a response to horrific massacres by Machar’s forces a year ago.

The accounts of the displaced people I interviewed are supported by a new United Nations report describing a “new brutality and intensity” to attacks in the area, citing nine separate instances in which government forces raped women or girls and then burned them alive in huts. A different United Nations assessment used satellite imagery to count some 250 structures burned in a single village, Ngop. The International Committee of the Red Cross has said that its compound in Leer was looted and that 100,000 fled that county alone.

“The violence against children in South Sudan has reached a new level of brutality,” warned Anthony Lake, executive director of Unicef, alluding to the army’s assault. “Survivors report that boys have been castrated and left to bleed to death. … Girls as young as 8 have been gang raped and murdered. … Children have been tied together before their attackers slit their throats.”Continue reading the main storyRECENT COMMENTSSarah Ruba July 5, 2015After reading most of the Readers’ Picks comments, I am disheartened. It seems that people are giving up on humanity, throwing up their...

D. H. July 5, 2015VOICE PROTEST After reading Nick Kristof’s horrifying piece, I hope that Obama will raise his voice in defense of victims in Sudan,...

Steve July 5, 2015I recommend for those criticizing Mr. Kristof to read the column a second time. He has not called for military intervention or boots on the...

SEE ALL COMMENTSOn his Africa trip, Obama should work closely with Kenya, Uganda and Ethiopia to impose targeted sanctions on the families of recalcitrant leaders in all factions, so they pay a price until there is peace. The United States has donated $1.1 billion in aid to South Sudan since the civil war began, but what is most needed isn’t money but tough, hands-on diplomacy to pressure all sides. Ethiopia has been trying to hammer out a peace, and it deserves more Western backing.

This is urgent, because the cycle of violence makes it ever more difficult to put South Sudan together again.

“I hate the Dinka now,” says Nyaluak Ngeach Tuak, whose 8-year-old son is missing and presumed dead.

Another woman, Nyabuol Rik Puol, says she wants counterattacks as vengeance against the soldiers who tried to rape her 11-year-old daughter and then killed her sister when she protested.

“What happened to us,” she told me venomously, “should happen to the Dinka, too.”