Politically charged street brawls broke out in Berkeley, California, on Sunday, with police arresting 13 charming participants on charges including assault with a deadly weapon. One Twitter video showed masked activists kicking a man curled in fetal position on the ground; the beat-down stopped only when a journalist, Al Letson, shielded the man with his body. “I was scared they were going to kill him,” Mr. Letson said.

As Charlottesville drew attention to the worst elements of the far right, Sunday’s melee revealed an increasingly...