Responding to Israeli threats, Ayatollah Ali Khamenei said Iran would show no mercy. If the threats turn out to be true, who is next in line after him?

Ayatollah Ali Khamenei has spent almost four decades as Shi'ite Iran's supreme leader building a regional power to rival the Sunni states across the Gulf and implacably hostile to the U.S. and Israel - while crushing repeated unrest at home.