He went by many names — Brother Wang, Mr Haha, Mr T, even Nelson Mandela. But to American agents, Zhi Dong Zhang was one thing only: the face of the world's fentanyl empire. The 38-year-old Chinese national, wanted by both the US and Mexico, was snared in Cuba after one of the biggest manhunts in modern narcotics history. Zhang, who stands just over five foot seven, small and wiry with an unkempt look, has been accused of flooding the world with synthetic poison — and of laundering millions through a web of ghost companies, secret accounts and offshore havens. Investigators...