A little more than two weeks ago, during a confrontation between protesters and law enforcement, an improvised explosive device was detonated on a public bridge in southern North Dakota. That was simply the latest manifestation of the “prayerful” and “peaceful” protests against the Dakota Access Pipeline.

Escalating tensions were temporarily defused Sunday when the U.S. Army Corps of Engineers, at the direction of the Obama administration, announced it would refuse to grant the final permit needed to complete the $3.8 billion...