America has long had a radioactive racist soup in the center of our national life. Donald Trump thinks he is stirring it for political benefit. He’s actually doing something more dangerous.

For much of our history, the soup was deadly and uncontained, spewing radiation that led to the enslavement, terrorization, murder and oppression of African-Americans. One hundred years ago this summer, it erupted on the streets of Washington, leaving dozens dead. A dean of Howard University narrowly escaped being lynched. The murders, beatings and threats erupted in countless places during the first 200 years of American life. To visit the National Museum of African American History and Culture is to discover that the violence and mistreatment is beyond count, but not beyond imagination.

Yet something good happened over the last 50 years. America started to get control over the dangerous radiation. We erected a containment building made up of laws; we passed statutes making the abuse and mistreatment of people by virtue of their race a crime. More important, we began enforcing the laws we already had. It was long a statutory crime to kill another human being; it just wasn’t against the law in practice to kill a black person in many places. The rights to vote and to equal treatment sounded muscular on paper, but they were weaklings in much of America. Slowly, slowly that began to change, through progress at ballot boxes and jury boxes, in police squad rooms and classrooms.

But the containment building of law was only part of the solution. Radioactivity lasts for centuries and it can still blow the lid off the building; true safety lies in control rods, pushed down into the soup to calm it, to cool it. Those control rods in America were cultural.