Drought and other conditions have interrupted the flow of water owed to the U.S. under a treaty.

The Tamaulipan thorn forest once covered 1 million acres on both sides of the US-Mexico border. Restoring even a fraction of it could help the region cope with the ravages of a warming world.

President Trump has threatened to slap new tariffs on Mexico for failing to deliver water under a decades-old treaty. But ...