On Saturday, June 21, the United States attacked three facilities in Iran critical to Tehran’s nuclear program. The attack, named Operation Midnight Hammer, included a strike package of seven B-2 bombers and over two dozen Tomahawk cruise missiles. At a Pentagon news briefing on Sunday, June 22, Chairman of the Joint Chiefs of Staff General Dan Caine stated that Operation Midnight Hammer involved over 125 U.S. aircraft to support the attack. This attack was the first time that the United States used its largest bunker-busting bomb, the GBU-57 Massive Ordinance Penetrator (MOP), in an operational conflict. General Caine emphasized that the United States dropped 14 MOPs on two targets. Iran does not currently possess a nuclear weapon. However, the International Atomic Energy Agency (IAEA) recently assessed that Iran had stockpiled enough uranium to construct nine nuclear bombs. Weaponization is much more difficult than stockpiling fissile material. If the attacks were successful, the June 21 attacks on Iranian nuclear facilities would prevent Iran from further stockpiling highly enriched uranium or enriching existing stockpiles to 90 percent uranium-235, which is most suitable for nuclear weapons. Q1: What facilities did the United States target? A1: Operation Midnight Hammer targeted two uranium enrichment facilities at Fordow and Natanz and one facility at Isfahan, which conducts several activities related to Iran’s nuclear program. Natanz and Fordow are the only operational enrichment facilities in Iran. However, on June 12, just before the first Israeli attacks, Iran’s Foreign Ministry and Atomic Energy Organization issued a joint statement that Iran had constructed a third enrichment facility that would begin operations soon. On June 19, the IAEA Chief Rafael Grossi stated that Isfahan was the most likely site for Iran’s future third uranium enrichment facility that Iran hinted at on June 12. The Natanz uranium enrichment facility was previously struck by Israel. On June 13, Israel took out significant portions of the Natanz centrifuge hall, power supply, and other support buildings. The IAEA also stated that the underground centrifuge hall had been hit by the June 13 attacks. The Isfahan nuclear complex conducts several key activities in Iran’s nuclear program. The nuclear campus includes a chemical laboratory, a uranium conversion plant to prepare uranium for enrichment, the Tehran reactor fuel manufacturing plant, a centrifuge manufacturing facility, and a metal processing facility. On June 13, the same Israeli attack also struck the chemical laboratory, conversion plant, fuel manufacturing plant, and metal processing facility. It is unclear what specific capabilities the recent U.S. attacks on June 21 with Tomahawk missiles were targeting. Copyright © 2025 Maxar Technologies Despite targeting Natanz and Fordow with B-2 bombers equipped with MOPs, the United States did not deploy these assets against Isfahan. Instead, it appears the United States only attacked Isfahan with Tomahawk cruise missiles. General Caine stated that “a U.S. submarine in the Central Command Area of Responsibility launched more than two dozen Tomahawk land attack cruise missiles against key surface infrastructure targets” at Isfahan. It also appears that the United States struck surface-to-air missile sites in Fordow prior to the B-2 attack. Isfahan may have more robust air defenses, making a B-2 operation more challenging. Alternatively, the targets attacked at Isfahan may not have required deep earth penetration capabilities. Nevertheless, satellite imagery reveals that the Tomahawk strikes at Isfahan dealt significant damage to several buildings at the nuclear complex. Q2: Why is Fordow so important? A2: Located deep within a mountain 29 kilometers north of the Iranian city of Qom, the Fordow uranium enrichment facility stands as a highly protected and strategically central site within Iran’s nuclear program. Its subterranean placement provides significant protection against potential aerial bombardment, a design choice indicative of its critical role. The facility is approximately 54,000 square feet and is believed to house 3,000 centrifuges. At Fordow, Uranium enrichment utilizes six cascades of older IR-1 centrifuges and ten cascades of more advanced IR-6 centrifuges. Under the terms of the 2015 Joint Comprehensive Plan of Action, Iran was explicitly prohibited from conducting enrichment activities at Fordow. However, following the U.S. withdrawal from the agreement in 2018 during the first Trump administration, Iran resumed enrichment activities at the site. Since then, the facility has notably been producing uranium enriched to 60 percent purity, a level far exceeding civilian needs for a peaceful nuclear energy program. Fordow was central to future Iranian nuclear weapons development. Of all of the nuclear targets struck by the United States, Fordow is the most challenging to destroy. Before the U.S. attack on June 21, reports indicated that only the United States possessed the conventional weapons capabilities required to cause severe damage to, or destroy, Fordow. The exquisite capability that the United States used on the attack was the MOP, a massive 30,000-pound bomb carrying 11 tons of TNT equivalent explosive power. Q3: How successful were the strikes? A3: There are conflicting reports about the success of the U.S. strikes on June 21. Immediately after the strikes, on June 21 at 10:00 p.m., President Donald Trump issued a statement calling the attacks “a spectacular military success” and emphasizing that “Iran’s key nuclear enrichment facilities have been completely and totally obliterated.” On June 22, Secretary of Defense Pete Hegseth stated in a Pentagon press conference that it was “an incredible and overwhelming success.” General Dan Caine, chairman of the Joint Chiefs of Staff, said that the attacks were designed “to severely degrade Iran’s nuclear weapons capability” but declined to comment on what nuclear capability remained at the sites. General Caine noted that battle damage assessments were still pending. On June 22, officials from the Israeli military told the New York Times that the Fordow nuclear site was substantially damaged, but not destroyed. The officials also indicated that Iran may have moved uranium away from the site before the attack. Regardless of whether the Fordow facility is completely destroyed, it is highly unlikely that enrichment activities will resume at Fordow in the near future. Copyright © 2025 Maxar Technologies Iran may have moved nuclear material and any stockpiled uranium out of Fordow in the days prior to the attack. However, Maxar satellite imagery from June 19 shows significant activity at Fordow consistent with defensive measures rather than material transport. This includes 16 dump trucks parked outside the underground tunnel entrances to the centrifuge facility. These dump trucks seem to indicate Iran taking defensive actions prior to the attack. Satellite imagery shows that all six entrances were sealed with rock and sand prior to the U.S. raid. An additional preventative measure that Iran could have taken is to flood the centrifuge hall with an inert gas to prevent contamination in the event of an attack. Uranium enrichment involves rapidly spinning uranium hexafluoride. When uranium hexafluoride comes into close contact with moisture in the air, it produces the toxic compounds uranyl fluoride and hydrogen fluoride. These compounds are poisonous if inhaled and would make it difficult to restart operations at Fordow. Q4: Where is Iran’s remaining uranium stockpiled? A4: The IAEA recently stated to the United Nations Security Council that Iran had amassed 400 kilograms of 60 percent enriched uranium. IAEA Director General Rafael Grossi told the New York Times that Iran had previously stored the nuclear material at Isfahan, presumably in an underground facility. U.S. Vice President JD. Vance indicated to ABC that Iran still controls the uranium stockpiles, and that the administration will work in the coming weeks to ensure that it does something with that fuel. This missing stockpile presents a critical proliferation risk. While Iran’s ability to enrich uranium has been severely degraded, the existence of this already 60 percent enriched material means that a significant barrier to weaponization has already been overcome. Without the operational enrichment facilities at Fordow and Natanz, Iran would theoretically need to acquire new centrifuges, repair the damaged centrifuges, or enrich at a covert enrichment facility to bring this 60 percent enriched uranium to weapons-grade levels. The longer the location of this highly enriched uranium stockpile remains unknown, the greater the potential for a proliferation crisis. It is unclear whether the material is currently at Isfahan or if it has been moved to one or multiple other locations. International diplomatic and intelligence efforts will undoubtedly intensify to locate and secure this material, underscoring the ongoing volatility of Iran’s nuclear program despite the recent strikes. Q5: What is next? A5: Initial statements from senior U.S. leadership, including President Trump, indicate that the attacks on Fordow, Natanz, and Isfahan were limited strikes and that the Trump administration does not want to enter a wider conflict. President Trump himself signaled that Iran “must now make peace.” Secretary of Defense Pete Hegseth also stated, “The United States does not seek war.” This suggests that the strikes were intended to address Iran’s nuclear program and are not the opening salvo for a wider engagement. Moving forward, Iran’s response will be critical. Initial reports indicate that Iran’s parliament has voted to close the Strait of Hormuz, although the ultimate decision rests with Iran’s Supreme National Security Council. If Iran were to attempt to close the Strait of Hormuz, it would effectively block a shipping route that accounts for approximately 20 percent of the world’s oil supply. In the past, under the first Trump administration, Iran pursued both retaliation and calibrated de-escalation. When the United States killed Qasem Soleimani in January 2020, Iran responded by attacking U.S. military bases in Iraq. However, in that instance, Iran reportedly gave plenty of advance warning, allowing U.S. forces to take cover and minimizing casualties. This past behavior suggests that Iran is capable of a measured response that seeks to avoid full-scale war. Joseph Rodgers is deputy director and fellow with the Project on Nuclear Issues at the Center for Strategic and International Studies in Washington, D.C. Critical Questions is produced by the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s). © 2025 by the Center for Strategic and International Studies. All rights reserved.