After the killing of Iran’s top nuclear scientist last week, US president-elect Joe Biden is coming under renewed pressure to quickly resume negotiations with the regime. He should slow down and proceed with caution.

Biden has long since telegraphed his desire to resuscitate the nuclear deal that Iran agreed to with the US and other world powers in 2015. The Joint Comprehensive Plan of Action, as it was known, was designed to pause Tehran’s nuclear-development programme well short of the weaponisation stage...