A federal judge on July 2 temporarily blocked Philadelphia from enforcing a city measure aimed at federal immigration operations, preventing the city from requiring federal officers to go unmasked, display visible identification, and use marked vehicles during enforcement activity. The ruling is an early but important development in a fast-evolving conflict between local efforts to regulate immigration tactics and the federal government’s claim to operational control over its officers.

At the center of the dispute is a familiar constitutional fault line: whether a municipality can impose rules that affect how federal officers carry out federal law.