Cannon Ball, North Dakota—For months, the Standing Rock Sioux Tribe has inveighed against plans to build a oil pipeline on land it claims to own. The U.S. government temporarily suspended construction on part of the pipeline in September. But the demonstrations have continued, as protesters like this one on horseback, remain undeterred. “The best way to resolve this,” says Dave Archambault II, the chairman of the Standing Rock Sioux, “is to reroute this pipeline.”