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.