If Israel was going to kill a top Iranian official, an act that had the potential to start a war, it needed the assent and protection of the United States. That meant acting before Mr. Biden could take office. In Mr. Netanyahu’s best-case scenario, the assassination would derail any chance of resurrecting the nuclear agreement even if Mr. Biden won.

The Scientist

Mohsen Fakhrizadeh grew up in a conservative family in the holy city of Qom, the theological heart of Shia Islam. He was 18 when the Islamic revolution toppled Iran’s monarchy, a historical reckoning that fired his imagination.

He set out to achieve two dreams: to become a nuclear scientist and to take part in the military wing of the new government. As a symbol of his devotion to the revolution, he wore a silver ring with a large, oval red agate, the same type worn by Iran’s supreme leader, Ayatollah Ali Khamenei, and by General Suleimani.

He joined the Revolutionary Guards and climbed the ranks to general. He earned a Ph.D. in nuclear physics from Isfahan University of Technology with a dissertation on “identifying neutrons,” according to Ali Akbar Salehi, the former head of Iran’s Atomic Energy Agency and a longtime friend and colleague.

He led the missile development program for the Guards and pioneered the country’s nuclear program. As research director for the Defense Ministry, he played a key role in developing homegrown drones and, according to two Iranian officials, traveled to North Korea to join forces on missile development. At the time of his death, he was deputy defense minister.

“In the field of nuclear and nanotechnology and biochemical war, Mr. Fakhrizadeh was a character on par with Qassim Suleimani but in a totally covert way,” Gheish Ghoreishi, who has advised Iran’s Foreign Ministry on Arab affairs, said in an interview.

When Iran needed sensitive equipment or technology that was prohibited under international sanctions, Mr. Fakhrizadeh found ways to obtain them.