Bernard Aronson listens as Secretary of State John F. Kerry speaks at the State Department on Feb. 20, 2015, announcing his appointment as special envoy for the Colombian peace process. Aronson stepped down this month when President Trump was inaugurated, as is routine, and the new administration has not said whether he will be replaced. (Brendan Smialowski/AFP/Getty Images)

Like most political appointees, Bernard Aronson, the U.S. special envoy for the Colombian peace process, was out of a job when the new administration took over this month.

Few American diplomats have been so central to the resolution of some of Latin America’s most intractable conflicts. Having helped negotiate the end of El Salvador’s civil war in 1992, Aronson, 70, was running a private investment firm when he was drafted back into service by the State Department in 2014. The Colombian government’s peace talks with FARC rebels were stalling, and President Juan Manuel Santos asked the United States for help.

It sent him “Bernie.”

The battle-hardened guerrilla commanders soon warmed to Aronson’s quiet pragmatism and disarming willingness to listen. Over two years and many trips to Havana, where the negotiations were held, Aronson helped troubleshoot the controversial accord with the Revolutionary Armed Forces of Colombia, or FARC, that is now in the early stages of implementation.

“He did a brilliant job in helping us get the FARC to understand the constraints of the real world,” said Sergio Jaramillo, Colombia’s peace commissioner, who was one of the government’s two principal negotiators, “and to agree to things that no guerrilla force has ever agreed to before in a negotiation: to be accountable before a tribunal for their war crimes, to repair their victims and to get out of the drug trade.”

Aronson leaves at a delicate time in Colombia’s effort to end its 50-year war, the longest in the Western Hemisphere. The peace-building phase has not gotten off to a great start, and the United States, which has given the country $10 billion in security aid since 2000, has a big stake in making sure the deal sticks.

There has been no indication the Trump administration will name a replacement for Aronson. Nor, for that matter, whether it will continue to support Santos, who was awarded the Nobel Peace Prize last year for his effort to extinguish a conflict that has killed more than 220,000.

“My job was to help them reach a peace agreement, and they’ve done that,” Aronson said in an interview. “I do think there is value in having someone senior in government focused on implementing the peace accord, and there’s some loss in not having someone who sees that as their responsibility.”

Aronson, who served as the top U.S. diplomat for Latin America under presidents George H.W. Bush and Bill Clinton, said he made clear to the guerrillas from the start that he was not a neutral party in the peace talks. His job was to represent the United States and support its ally, the Colombian government. But over time, he developed a personal rapport with FARC commanders that no other U.S. diplomat has had.

Aronson said he was not sure what would happen with his post. Analysts were more blunt.

“Losing the special envoy will leave a vacuum,” said Adam Isacson, a Colombia expert at the Washington Office on Latin America.

In written responses he provided to senators, Rex Tillerson, Trump’s nominee for secretary of state, said the United States should review Colombia’s peace accord and decide “to what extent” it will continue to support it.

Many Republicans and Democrats view the agreement as a vindication of Plan Colombia, the counternarcotics and counterinsurgency initiative credited with tipping the conflict in the government’s favor.

After a decade of devastating losses, including the deaths of several key leaders in airstrikes enabled by U.S. military technology and intelligence, the FARC came to the bargaining table in 2012 to negotiate the terms of its disarmament and transformation into a peaceful political party.

Enrique Santiago, a Spanish attorney who was one of the FARC’s lead negotiators, said Aronson “knew how to build a relationship of trust with both sides.”

“He provided sage advice at the most difficult moments of the talks,” Santiago said, “while always remaining a staunch defender of U.S. interests.”

But the Obama administration’s central role in the peace deal has raised concerns that the new administration could view it the same way it sees the Obama-backed opening to Cuba — as a political trophy of the previous regime. Aronson said he hoped that would not happen.

“If you think about the conventional wisdom on U.S. foreign policy — that we can’t do anything on a bipartisan basis, that we have a short attention span, and we fail when we try to build democratic institutions — Colombia refutes all that,” Aronson said.

“We’ve sustained our commitment and our support for Colombia, and we’ve helped it become the most vibrant democracy in South America,” he said. “That’s what I hope [the Trump administration] will build on.”

As president, Barack Obama proposed a 40 percent increase in 2017 U.S. assistance, to $450 million, relaunching Plan Colombia as “Peace Colombia,” with funds for the removal of land mines, crop substitution in coca-growing zones and economic development in rural areas.

But Congress has yet to approve the funds, and some conservative U.S. lawmakers have signaled they want to withhold U.S. aid unless the government and the FARC reopen the peace agreement.

In a recent interview with the Washington Times, Rep. Mario Diaz-Balart (R-Fla.) called the accord “unacceptable” and a potential path to a guerrilla takeover of Colombian electoral politics.

“We are going to be attaching more and more strings to make sure that the money does not go to handing Colombia over to FARC,” Diaz-Balart said.

U.S. officials have been frustrated by a rebound in Colombia’s cocaine output, as the size of the country’s illegal coca crop has doubled in the past two years. As FARC units withdraw from drug-growing areas long under their control, other armed groups have been muscling their way in.

Colombia’s government blames those groups for an increase in lethal attacks on community leaders, leftist activists and others who sympathize with FARC and support the peace accord. The killings are viewed as a major threat to the agreement’s successful implementation.

The peace process is already stumbling. FARC’s roughly 6,000 fighters and thousands of irregular “militia” members are supposed to begin moving into U.N.-monitored camps to disarm. But preparation of the camps has been behind schedule.

In the meantime, the government has also launched formal talks with leftist ELN guerrillas, Colombia’s smaller insurgent group. The United States will not have a special envoy at those talks unless a replacement is named for Aronson, who has returned to running the Washington-based private equity firm he co-founded.

“I don’t think the Trump administration will oppose peace implementation” in Colombia, said WOLA’s Isacson. “Still, it’s likely that U.S. government support for the peace effort is about to become much less enthusiastic.”