During the campaign, Mr. Trump declared that “torture works,” and he vowed to “immediately” reinstate techniques like waterboarding because “we have to beat the savages” of the Islamic State, who “deserve” such treatment even if it is fruitless. Since the election, Mr. Trump has indicated that he might be reconsidering his position, citing the firm stance against torture by James N. Mattis, his Pentagon nominee.

In an interview with The New York Times in November, Mr. Trump said that he was “surprised” when Mr. Mattis told him that he opposed torture and instead favored more humane interrogations of prisoners based on rapport building.

But Mr. Trump did not close the door entirely. If Americans feel strongly about bringing back waterboarding and other tactics, he said, “I would be guided by that.”

Nora Sveaass, a psychologist at the University of Oslo and a former member of the United Nations Committee Against Torture, warned that if Mr. Trump revived the use of torture by the United States, it would have a ripple effect around the globe. “The U.S. is a very strong voice,” Ms. Sveaass said.

“It’s just like putting a bomb into all of those major principles — the absolute prohibition on torture; the absolute obligation to provide redress and justice to victims of torture, including rehabilitation; the obligation to investigate and hold people to account,” she added. “If one country such as the U.S. openly torpedoes those principles, you can just forget about asking for compliance from states already challenging the absolute prohibition.”

The signal from Mr. Trump that torture is acceptable again comes just as countries from Argentina to Tunisia, either through courts or special truth commissions, are engaged in tentative efforts to hold themselves accountable for past conduct.

In Argentina, Omar Graffigna, the 90-year-old former chief of the country’s air force, was sentenced to prison in September for the 1978 kidnapping and torture of two left-wing activists, Patricia Roisinblit and José Manuel Pérez Rojo. The prosecution of Mr. Graffigna was just one in a series of old cases that have been brought into the courts this year in Argentina, as the nation comes to grips with the legacy of its “dirty war” of the 1970s and early 1980s.