A water war is looming between Mexico and the US. Neither side will win

Tensions are rising in a border dispute between the United States and Mexico. But this conflict is not about migration; it’s about water.

Under an 80-year-old treaty, the United States and Mexico share waters from the Colorado River and the Rio Grande, respectively. But in the grip of severe drought and searing temperatures, Mexico has fallen far behind in deliveries, putting the country’s ability to meet its obligations in serious doubt.

Some politicians say they cannot give what they do not have.

It’s a tough argument to swallow for farmers in South...