Rodrigo Duterte last week imposed martial law for 60 days over the entire island of Mindanao in response to terrorist groups taking control of parts of Marawi City. The Philippine President’s decree may help the armed forces retake the town and drive out militants who flew the black flag of Islamic State. But his use of special powers deserves close scrutiny in light of his past disrespect for the constitution.

Abu Sayyaf and the Maute Group, the two Islamist gangs responsible for the violence, are known for their brutality....