Nearly 75 years ago, an American president was eyeing a grand project along our southern border, not to divide the United States and Mexico but to bring the two nations together. On June 12, 1944, a week after D-Day, President Franklin Roosevelt signed legislation establishing Big Bend National Park, almost a million acres along the Rio Grande in West Texas.

He followed up with a grand challenge to President Manuel Ávila Camacho of Mexico: “I do not believe that this undertaking in the Big Bend will be complete until the entire park area in this region on both sides of the Rio Grande forms one great international park.” Mr. Camacho agreed.

Still, the building of a great international park along our southern border, rather than a grim medieval wall, remains an elusive goal. But if there ever was a moment for it, this is it, and particularly in a place where time and the flowing river have already carved truly great walls along thousand-foot-deep canyons.