As President Trump continues his mad purge of the Department of Homeland Security, a sinister figure is emerging as the driving force behind all the chaos: Stephen Miller, the president’s top immigration adviser.

A fierce restrictionist, Mr. Miller seems to have rarely met an immigrant he didn’t want to deport. He is among the hardest of hard-liners, known for spurring his boss to pursue ever more draconian measures.

Not that Mr. Trump needs much encouragement. He did, after all, ascend to the White House on an immigrant-bashing, ethnonationalist platform as dishonest as it was divisive. Many of the administration’s uglier policy ideas have been a result of Mr. Trump’s failure to deliver on his signature campaign promise of a “big, beautiful” border wall. Small wonder Mr. Miller has emerged as his favorite consigliere on such matters.

Mr. Miller’s preferred approach to immigration and border security is straight out of Ann Coulter’s fever dreams. He was a chief architect of the travel ban on citizens from Muslim-majority countries. He favors the building of tent cities at the border for warehousing asylum seekers. He’s agitating to end the 20-day limit on detaining migrant children. And he wants to reinstate the practice of snatching migrant children from their families — though this time he’d like to give parents a “binary choice” of having their kids taken from them or held with them in detention indefinitely.