Why did a judge grant Trump’s request for a ‘special master’?

Monday’s decision by a US federal judge to stop the investigation of Donald Trump’s seizure of classified documents until a “special master” reviews them has thrown the political and legal worlds into uproar – pitting those who believe the ex-president has special legal privileges against those who think he should be treated like any other citizen.

The procedural victory for Trump means the Department of Justice will have to press pause on its effort to determine whether the former president may have illegally run off with classified documents from the...