What happened in Philippine drug war that led to Duterte's arrest?

MANILA (Reuters) -Former Philippine President Rodrigo Duterte was arrested and taken into custody on Tuesday in Manila on an International Criminal Court warrant for the "war on drugs" that defined his term in office and which killed thousands of Filipinos. As longtime mayor of the southern Philippine city of Davao, Rodrigo Duterte was known as "the punisher" for his harsh policies.