He is the son of Ponca Tribe activists from Oklahoma who took him to rallies when he was a baby. Mr. Horinek, 43, remembers riding on his father’s and uncle’s shoulders as they marched with Cesar Chavez in the California fruit-pickers’ protests.

“I can’t remember a time that I wasn’t being taught to stand up for human rights, native rights,” he says.

He came to North Dakota for a cause. Here is how he describes that cause now:

“What I said to the police officers when I was sitting down in a prayer circle, I asked them, ‘Don’t you drink water, too?’” he says. “Don’t your children drink water? We’re here to protect the water. This isn’t just a native issue. We’re here protecting the water, not only for our families and our children, but for your families and your children. For every ranch and every farm along the Missouri River.”

Law enforcement officials have accused the protesters of rioting and attacking pipeline contractors, and they have arrested more than 400. But Mr. Horinek says the protesters — water protectors, they call themselves — are not the bad guys. He tells a story:

Last week, he and 49 other demonstrators decided to link arms and sit together by the overturned earth where the Dakota Access pipeline is slated to go. They were on what the Standing Rock Sioux Tribe considers sacred ancestral land, but from a legal perspective, it is owned by the pipeline company. So sheriff’s officers arrested them for trespassing.