Legal marijuana may be doing at least one thing that a decades-long drug war couldn't: taking a bite out of Mexican drug cartels' profits.

The latest data from the U.S. Border Patrol shows that last year, marijuana seizures along the southwest border tumbled to their lowest level in at least a decade. Agents snagged roughly 1.5 million pounds of marijuana at the border, down from a peak of nearly 4 million pounds in 2009.

The data supports the many stories about the difficulties marijuana growers in Mexico face in light of increased competition from the north. As domestic marijuana production has ramped up in places such as California, Colorado and Washington, marijuana prices have fallen, especially at the bulk level.

"Two or three years ago, a kilogram [2.2 pounds] of marijuana was worth $60 to $90," a Mexican marijuana grower told NPR news in December 2014. "But now they're paying us $30 to $40 a kilo. It's a big difference. If the U.S. continues to legalize pot, they'll run us into the ground."

And it's not just price — Mexican growers are facing pressure on quality, too. "The quality of marijuana produced in Mexico and the Caribbean is thought to be inferior to the marijuana produced domestically in the United States or in Canada," the DEA wrote last year in its 2015 National Drug Threat Assessment. "Law enforcement reporting indicates that Mexican cartels...