Even with the high-profile arrests of suspects by masked immigration agents and the plane loads of migrants swiftly ferried out of the U.S., President Trump was falling short of the number of daily deportations carried out by the Biden administration in its final year. So in late May, Stephen Miller, a top White House aide and the architect of the president’s immigration agenda, addressed a meeting at the headquarters of Immigration and Customs Enforcement, known as ICE. The message was clear: The president, who promised to deport millions of immigrants living in the country illegally, wasn’t pleased. The agency had...