The children died one after the other. Twelve acutely malnourished infants living in one corner of Sudan’s war-ravaged capital, Khartoum.

Abdo, an 18-month-old boy, had been rushed to a clinic by his mother as he was dying. His ribs protruded from his withered body. The next day, a doctor laid him out on a blanket with a teddy bear motif, his eyes closed.

Like the other 11 children, Abdo starved to death in the weeks after President Trump froze all U.S. foreign assistance, said local aid workers and a doctor. American-funded soup kitchens in Sudan, including the one near Abdo’s house, had been the only lifelines for tens of thousands of people besieged by fighting.

Bombs were falling. Gunfire was everywhere. Then, as the American money dried up, hundreds of soup kitchens closed in a matter of days.

“It was catastrophic,” said Duaa Tariq, an aid worker.

The stark consequences of Mr. Trump’s slashing of U.S. aid are evident in few places as clearly as in Sudan, where a brutal civil war has set off a staggering humanitarian catastrophe and left 25 million people — more than half of the country’s population — acutely hungry.