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The conviction this week of the Mexican crime lord Joaquín Guzmán Loera was one of the most visible victories for American law enforcement since the war on drugs began in the 1970s, a triumph over a cartel leader who survived — and thrived — for decades on his business skills, brutal violence and bottomless bribes to Mexican officials.

And yet on Jan. 31, the same day that the trial of Mr. Guzmán — known to the world as El Chapo — ended in a Brooklyn federal courtroom, border officials in Arizona made an announcement: They had just seized the largest load of fentanyl ever found in the United States, a haul that was hidden in a truck carrying cucumbers on its way through the Nogales port of entry, a crossing that Mr. Guzmán’s organization, the Sinaloa drug cartel, has traditionally run for years.

The fentanyl seizure — enough for 100 million lethal doses — was a clear signal that even after the hard-fought task of convicting Mr. Guzmán on drug conspiracy charges, American federal agents have far to go in their attempts to dismantle Mexico’s infamous cartels. The verdict against the kingpin on Tuesday may, in the end, have little lasting impact on either Mr. Guzmán’s group or the wider effort to stem the flow of drugs into the United States.

Even without its former leader, the Sinaloa cartel is a major threat and maintains among its competitors “the most expansive footprint in the United States,” according to the Drug Enforcement Administration’s most recent assessment of the drug trade. While Mr. Guzmán’s guilty verdict disrupted the relationships and smuggling agreements the kingpin forged in his career, federal agents say his empire remains intact and is now being run by his sons and his wily longtime partner, Ismael Zambada García.