When Colombia signed a landmark peace agreement with rebels in 2016, it was celebrated internationally for ending a war that had ravaged much of the country for decades. The United States bolstered the peace efforts, helping displaced farmers return to their land and helping prosecute war crimes.

Now, support from the U.S. government — the agreement’s biggest foreign economic backer — has vanished.

As the Trump administration has withdrawn most foreign assistance globally, including dismantling the U.S. Agency for International Development, it has undercut a deal designed, in part, to curtail the flow of drugs to the United States.

“This puts wind in the wings of armed groups,” said León Valencia, director of the Bogotá-based Peace and Reconciliation Foundation, an organization that works on post-conflict issues and had received U.S. funds. “They can tell demobilized guerrillas or victims that the government signed a peace agreement and didn’t keep its promise.”

Since 2001, U.S.A.I.D. has spent more in Colombia than any other South American country, about $3.9 billion.

León Valencia, director of the Peace and Reconciliation Foundation, at his home in Bogota, Colombia, on Sunday. The withdrawal of U.S. funds “puts wind in the wings of armed groups,” he said.Credit…Nathalia Angarita for The New York Times