President Trump is not generally known as a student of history. But on Tuesday, during a combative exchange with reporters at Trump Tower in New York, he unwittingly waded into a complex debate about history and memory that has roiled college campuses and numerous cities over the past several years.

Asked about the white nationalist rally that ended in violence last weekend in Charlottesville, Va., Mr. Trump defended some who had gathered to protect a statue of Robert E. Lee, and criticized the “alt-left” counterprotesters who had confronted them.

“Many of those people were there to protest the taking down of the statue of Robert E. Lee,” Mr. Trump said. “So this week, it is Robert E. Lee. I noticed that Stonewall Jackson is coming down.”