Why It Is So Difficult to Stop the Flow of Fentanyl Into the U.S.

When President Trump threatened to impose steep tariffs on Mexico, Canada and China, he blamed these three countries for enabling the flow of fentanyl into the United States and fueling what officials have called a national emergency.

President Claudia Sheinbaum of Mexico swiftly pushed back on Mr. Trump’s suggestions that her government colluded with drug traffickers, calling it “slander.” And she also put the blame on the United States, arguing that the fentanyl crisis stemmed from immense domestic demand for drugs, including fentanyl, and the illegal sale of U.S. guns to cartels.

Ms. Sheinbaum announced Monday that she had reached an agreement with Mr. Trump, who agreed to pause the tariffs as she promised to send 10,000 members of the National Guard to the U.S.-Mexico border to stop the trafficking of fentanyl.

But thwarting the transport of the deadly synthetic opioid into the United States poses significant challenges for both the United States and Mexico given the cartels’ immense resources, the ease with which fentanyl is produced and moved and the insatiable demand for narcotics among U.S. consumers, analysts and experts say.

Given these factors, it may be extremely difficult for Mexico to demonstrate that it is meeting Mr. Trump’s terms — particularly within the 30-day window he allotted to delay the imposition of tariffs.