The assassination last week of Mohsen Fakhrizadeh, Iran’s most important nuclear scientist, was aimed as much at the administration of U.S. President-elect Joe Biden as it was the regime in Tehran. The murder raises tensions in the Persian Gulf region as Iranian authorities threaten retaliation and nationalist Iranians demand action. The killing raises legal, moral and strategic questions, all of which must be addressed as the political context in which those decisions must be made is being transformed.

Fakhrizadeh was one of the architects of Iran’s nuclear program and head of its bomb-building effort, one that was reportedly halted in 2003, but was suspected to have continued in secret. Israeli Prime Minister Binyamin Netanyahu identified him by name in a 2018 presentation, calling him national enemy No. 1.

Fakhrizadeh was killed in an ambush while traveling outside Tehran. While no group or government has taken credit for the killing, it is generally assumed that Israel is responsible; details are murky, but the strike has the hallmarks of an Israeli operation. Iran’s foreign minister, Javad Zarif, called the killing “an act of state terror,” while President Hassan Rouhani promised retaliation “at the proper time,” and accused Israel of being behind the attack. Supreme Leader Ayatalloh Ali Khamemei pledged “definitive punishment of the perpetrators and those who ordered it.”