Foreign defendants brought to the United States almost never face capital punishment, no matter how grave the allegations against them.

But when a notorious drug lord arrived from Mexico in Brooklyn federal court last month on charges that included killing a federal agent, prosecutors for the Eastern District of New York said that he might face the death penalty.

Prosecutors would still have to formally seek capital punishment for the drug lord, Rafael Caro Quintero, in advance of a trial that could be months or years away. But whatever becomes of Mr. Caro Quintero, the episode represents a sea change for both countries, reflecting how Mexico is responding to President Trump’s aggressive foreign policy in the Americas and beyond.

Before this, Mexico had historically released criminals to the United States only on the condition that they not be executed, a provision of its extradition agreement with Washington.

However, rather than going through the cumbersome extradition proceedings, Mexico simply expelled Mr. Caro Quintero and 28 other drug cartel figures, as allowed by a national security law. The measure gives the Mexican government flexibility to speed up removals and it means that Mr. Caro Quintero and at least four other prisoners sent north last month could also face the death penalty.

For Mexico, the decision is a break from the country’s longstanding policy of protecting its citizens from capital punishment. For the United States, it enables Mr. Trump’s punitive vision of justice, of which the death penalty is an essential tool.

Mexico has fought bitterly for decades to stop the U.S. government from executing its citizens. The extradition treaty, a form of which has been in place since the 1970s, stipulates that whichever country requests a defendant cannot impose the death penalty if it is not present in the defendant’s home country. Mexico has not used capital punishment since the 1960s, though it wasn’t officially abolished until 2005.

The two countries’ differing views have strained relations. In 2002, Mexico’s president, Vicente Fox, canceled a trip to visit President George W. Bush in protest of the impending execution of a Mexican citizen. In 2003, Mexico appealed to the United Nations’ highest court over death sentences that the U.S. government had imposed on 51 Mexican citizens.

In 2017, Mexico agreed to extradite the drug lord Joaquin Guzmán Loera, known as El Chapo, under the condition that Eastern District prosecutors not pursue the death penalty. He was sentenced to life in prison in 2019.

Emily Edmonds-Poli, a professor of political science and international relations at the University of San Diego, said that the decision of Mexico’s new president, Claudia Sheinbaum, to expel the cartel members would ordinarily carry political risk. But Ms. Sheinbaum, who is enjoying high approval ratings amid a wave of nationalism, may have the freedom to act boldly, she said.

“It’s a watershed moment,” Ms. Edmonds-Poli said. “It opens a door that had previously been firmly shut.”

Ms. Sheinbaum’s predecessor, Andrés Manuel López Obrador, sought to end violence through less confrontation with the cartels and addressing root causes. But his strategy, coined “hugs, not bullets,” has fallen out of favor in Mexico.

By contrast, Ms. Sheinbaum has so far adopted a decidedly more aggressive approach to fighting the cartels. In addition to approving the expulsions, she sent more than 10,000 troops to the U.S. border and to Sinaloa, a hub for fentanyl trafficking where her administration says it has made more than 900 arrests since October.

It is not clear how the Mexican government will respond should U.S. prosecutors seek the death penalty against the cartel members. Alejandro Gertz Manero, Mexico’s attorney general, told reporters in Mexico that the cartel bosses cannot be executed in the United States, as reported by the Spanish-language outlet El País.

Negotiations to have the drug lords expelled from Mexico under this streamlined process began during the Biden administration, according to two people familiar with the talks. The Biden White House renewed those discussions with Ms. Sheinbaum when she took office in October, and the final expulsion deal was hashed out by the Trump administration after Inauguration Day.

“It’s a short circuiting of an important legal procedure,” said Austin Sarat, a professor at Amherst College who has studied the death penalty for decades. “What Trump is doing is resetting the conversation around capital punishment.”

Mr. Caro Quintero was a particularly prized catch for American prosecutors. He was convicted in Mexico for orchestrating the 1985 torturing and killing of Kiki Camarena, an undercover agent with the Drug Enforcement Administration, which transformed the agency and U.S.-Mexico relations.

Mr. Caro Quintero served decades in Mexican prison, but was released in 2013 in the middle of the night thanks to a legal loophole. He was recaptured by the Mexican authorities in 2022. Michael Vitaliano, a lawyer for Mr. Caro Quintero, said in a statement that should his client face the death penalty, his legal team was “fully prepared to meet that challenge procedurally and substantively,” from “the moment of his seizure and expulsion from Mexico to the end of trial.”

It could be months before prosecutors announce whether they are seeking the death penalty. A spokesman for the Eastern District declined to comment.

Prosecutors would first have to clear hurdles, including an intense review inside the Eastern District office and a Justice Department committee in Washington that considers capital cases. During this time, defense attorneys may make appeals to prosecutors and then to the Washington committee.

Opponents of the death penalty have long pointed to racial disparities in its application, along with the more fundamental moral question of whether the state has the right to take a life.

Critics have also pointed to the high cost of administering the death penalty, which can be tens of thousands of dollars more expensive than life imprisonment, as well as the fact that the United States executes far more people than countries in its peer group. Among the 38 countries in the Organization for Economic Cooperation and Development, the United States and Japan are the only two that use the death penalty.

Ken Montgomery, a lawyer for Vicente Carrillo Fuentes, another cartel member expelled from Mexico who could face death, said in an interview that the United States should not be in the business of executing people.

“For a civilized society, I don’t think executing people is ever a civilized thing to do,” Mr. Montgomery said.

Just over half of Americans support the death penalty, according to an October poll from Gallup, compared with 80 percent three decades ago. Nationally, 25 people were executed in 2024, compared with 85 in 2000, according to the Death Penalty Information Center. President Joseph R. Biden Jr., who campaigned in 2020 on ending capital punishment, placed a moratorium on federal executions and commuted the sentences of 37 out of 40 inmates on death row before leaving office.

By contrast, Mr. Trump and his allies favor a more punitive approach to administering justice, with Mr. Trump himself long harboring an affinity for the death penalty. In 1989, he placed newspaper advertisements calling for New York State to adopt the death penalty after the brutal attack of a Central Park jogger, for which five Black and Hispanic teenagers were wrongfully convicted. (The ads did not directly call for the execution of the teenagers.)

In 2017, shortly after an Uzbek terrorist, Sayfullo Saipov, drove a truck through a crowded bike path in Lower Manhattan, killing eight people, President Trump said on Twitter that Mr. Saipov “SHOULD GET DEATH PENALTY!” During his first term in office, Mr. Trump restarted federal executions after a 20-year pause. And throughout his 2024 campaign, Mr. Trump said that “drug dealers and human traffickers” should be put to death.

In January, Mr. Trump signed an executive order calling for the death penalty in cases involving “the murder of a law enforcement officer” and “a cap­i­tal crime com­mit­ted by an alien illegally present in this country.”

In a Feb. 5 memo, Pam Bondi, the attorney general, lifted the moratorium that Mr. Biden had placed on executions.

Alan Feuer contributed reporting.