MANILA—At the center of a gunbattle and hostage standoff between Islamic State-linked militants and government troops in the southern Philippines is a faction leader who is trying to build a caliphate in the troubled region.

Isnilon Hapilon, 51, leads a branch of the Abu Sayyaf group known for kidnapping and beheading foreign tourists. His faction, which President Rodrigo Duterte has pledged to destroy, has preoccupied the Philippine military since 2014, when he publicly declared loyalty to Islamic State.

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