Late one winter morning five years ago, a polo-playing aristocrat arrived at an upmarket central London hotel, hoping to end the most savage matches he’d played. Former Inter-Services Intelligence deputy chief Major General Sahibzada Isfandiyar Pataudi had been hand-picked for the secret mission to meet with the man who had long served as India’s goalkeeper against Pakistani intelligence. Ethnic Tamil, but Punjabi-speaking, the General’s dour companion commanded the Research & Analysis Wing’s operations against jihadists in Pakistan. Then-army chief General Qamar Javed Bajwa, Pataudi explained at the first of their three meetings, was committed to ending the ISI’s secret war...