Since the late 1990s, more than 18 police commanders have left the New York City police department to run their own agencies elsewhere. This unprecedented migration has spread the Compstat revolution—the data-driven transformation of policing begun under New York police commissioner William Bratton in 1994—across the nation. Some of the transplants are well-known: Bratton himself now heads the Los Angeles Police Department; and his former first deputy, John Timoney, has led both the Miami and the Philadelphia forces. But the diaspora also includes lesser-known young Turks who rose quickly through the NYPD’s ranks during the paradigm-shattering 1990s. Now, as chiefs...