The June 2025 war between Israel and Iran, called the 12-Day War, saw the death of many Iranian nuclear scientists who participated in or are linked to Iran’s nuclear weapons program.  The Israel Defense Forces (IDF) has listed the names of eleven nuclear scientists that were eliminated during a series of attacks on the opening night of the war and afterwards.  Media reporting has identified several other scientists who died during the war, bringing the total to almost 20 scientists.  In addition, many family members were killed in the attacks.

Overall, the elimination of these nuclear scientists deprived Iran’s nuclear weapons program of its most capable and experienced personnel.  This act weakened Iran’s base for building nuclear weapons, eliminating needed expertise and hard-to-get management experience.

Iranian officials were quick to respond to the killings of the scientists, one proclaiming, "Hundreds of nuclear scientists can replace those who were martyred," according to Tasnim News Agency (affiliated to the IRGC).  Following the assassination of five nuclear scientists from Shahid Beheshti University (SBU), according to Tasnim News Agency, its President stated that “We have lost scientists, and this loss created a gap, and restoring it will take time. They were key players who are gone from us today, but it doesn’t mean we cannot fill their places.”

This is not the first time Israel has targeted scientists associated with Iran’s nuclear weapons program.  In total, six were killed prior to the 12-Day War, mostly in the period 2010-2012 and in 2020.  These were relatively isolated attacks, disruptive in the short term, but losses were manageable in the long term for Iran, in the way the President of the University of Beheshti alludes to.

However, this time the Israeli effort is different and recovering may be far more difficult and take far longer. Not only were killings in the 12-Day War on a much larger scale, they were also part of a broader Israeli program. In an apparent effort to pre-empt recovery and recruitments, Israel threatened a far larger group of scientists during the war via social media, an effort that may continue, warning them explicitly that death awaits them if they work on nuclear weapons.  They reached out throughout Iran offering rewards and safety to informants who provide information about secret nuclear activities.  The desired message is clear: Any Iranian scientist or engineer who decides to work on nuclear weapons or on secret gas centrifuge programs will know that his or her life, and potentially their family’s lives, are at risk and that a colleague nearby could become an informant, exposing the entire secret effort, with potentially devastating consequences.

In addition, Israel also targeted the detailed nuclear weapons information, designs, and data needed to develop and build nuclear weapons.  It destroyed a copy of the Nuclear Archive located in the basement of the project’s headquarters in Tehran, apparently in the Organization of Defensive Innovation and Research (SPND) headquarters.  It also destroyed or severely damaged a range of nuclear weaponization development and production facilities, likely laden with sensitive equipment and data.

When discussing the attack of the Iranian nuclear program, many cite the phrase “knowledge cannot be destroyed.” But it is well known through history that it can be forgotten, lost, or suppressed.  In a highly secretive program such as Iran’s nuclear weapons program, highly cognizant of the risk of leaks, it is likely that full knowledge of the most sensitive, most current developments of the program and how individual parts were intended to work together existed only in the heads of a few. Those Iranians who choose to work on producing nuclear weapons in the future will possess less information and expertise, while facing increased personal risks, resulting in an overall notably greater challenge in building the weapon itself.

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See full text at the Institute for Science and International Security: Significance of the Targeted Nuclear Scientists in the 12-Day War