Just a few days into the new year, 2020 began with high tensions between the United States and Iran. Kicked off by a US airstrike that killed a leading Iranian general and followed by Iranian missile strikes on bases in Iraq housing US troops, many feared that military conflict could be imminent. One question that raised particular alarm was the prospect that nuclear weapons might be involved. The situation has, fortunately, calmed down, but confusion about the relationship between Iran’s nuclear power program and its ability to build a nuclear weapon, as well as US options for using nuclear weapons against Iran, remains.
Here are some of the most important things to understand about what role nuclear weapons could play in a US-Iran conflict:
Q: Does Iran have nuclear weapons?
A: No, Iran does not possess nuclear weapons. It is a non-nuclear weapon state under the Nuclear Nonproliferation Treaty (NPT) and is a member in good standing under this treaty. Iran’s declaration that it will no longer abide by some of the provisions of the 2015 Iran nuclear deal does not change this status, because those provisions go beyond what is required under the NPT.
Q. Did Iran have a nuclear weapons program in the past?
A: Yes. The NPT guarantees non-nuclear states the right to acquire nuclear reactors and related facilities for civilian use but requires that states inform the International Atomic Energy Agency (IAEA) in advance about any nuclear facilities that it plans to build so the agency can establish safeguards (verification methods) to verify their peaceful use. For example, this is important for enrichment facilities; these produce low-enriched uranium for fuel, but can also be used to produce highly enriched uranium for nuclear weapons.
In the early 2000s, it became clear that Iran had been withholding information about its enrichment activities and in 2005, the IAEA reported to the UN Security Council that Iran was not in compliance with its IAEA Safeguards Agreement. The Council passed a resolution in 2006 requiring Iran to stop its uranium enrichment activities, and when Iran refused to do so, it imposed UN sanctions on Iran, as did the United States.
It was later revealed that Iran had a secret nuclear weapons program, which ended in 2003. The program was in an early stage, and Iran did not produce a nuclear weapon.
To assure the international community it no longer had a weapons program, under the 2015 Joint Comprehensive Plan of Action (JCPOA, or Iran nuclear deal), Iran accepted more stringent IAEA verification measures as well as restrictions on its civilian nuclear activities, which are not required by the NPT. In response, the nuclear-related sanctions were lifted in January 2016.
Q: How soon could Iran develop a nuclear weapon if it chose to do so?
A: This depends on what Iran decides to do next. Iran has the technical expertise necessary to develop a nuclear weapon if it chooses to do so, but it is not clear how much additional research it would need to be able to produce a useable weapon if it resumed its bomb program. However, a limiting factor is acquiring enough plutonium or highly enriched uranium needed for a nuclear weapon.
Prior to the 2015 nuclear deal, the Obama administration estimated that it could take as little as 2-3 months for Iran to produce enough highly enriched uranium for a nuclear weapon. The Iran nuclear deal was specifically designed to lengthen this “breakout” time from 2-3 months to at least a year. It also put in place intrusive verification measures to give other states confidence that Iran was complying with the agreement— or to quickly detect any attempts at cheating, giving the international community more time to respond.
In May 2018, President Trump unilaterally withdrew the United States from the Iran nuclear deal and re-imposed sanctions, despite repeated confirmation from the IAEA that Iran remained in compliance with the agreement. While the deal—which also includes Russia, China, Britain, France, Germany, and the European Union—remains in place, Iran has, over time, gradually weakened its compliance with key restrictions. In May 2019, Iran announced that it would no longer abide by some limits, including on enriched uranium reserves, and since then it has taken additional steps to lessen its compliance with other aspects of the deal. Most recently, on January 5, 2020, following the US killing of Iranian general Qasem Soleimani, Iran stated that it would no longer comply with the limit on its number of centrifuges, which are used to enrich uranium.
Iran has said, however, that it will continue to accept the stringent verification methods of the Iran nuclear deal and fully cooperate with the International Atomic Energy Agency. This will allow continued monitoring of any increase in the numbers of its two types of centrifuges as well as its stockpile of enriched uranium. Iran has also noted that all the steps it has so far taken are reversible if the United States ends its sanctions.
Q: Can the United States legally use a nuclear weapon against Iran?
A: No. All US military actions—including the use of nuclear weapons—are constrained by the rules of war. The world saw this play out in early January 2020 when President Trump tweeted on January 4 that the United States had targeted 52 Iranian sites, including significant cultural sites. Secretary of Defense Mark Esper then publicly stated that attacking these sites would be illegal and that the military would not carry out an illegal order from the president.
Any nuclear attack on Iran would also be illegal. The Law of Armed Conflict requires that any use of force comply with three basic principles: military necessity (attacks must be limited actions necessary to accomplish legitimate military objectives); distinction (attacks must discriminate between military and civilian targets); and proportionality (the military objective must outweigh the harm caused to civilians). The use of nuclear weapons against Iran fails on all counts. The United States has the most powerful conventional military in the world and could achieve any “legitimate military objective” by using its conventional weapons.
All members of the military must uphold these principles, even if it requires disobeying orders. Any military member who violates this requirement can be court-martialed, and military leaders could be charged with crimes against humanity and tried as war criminals in international fora.
While less restrictive, attacking Iran with a nuclear weapon is also counter to US policy on the first use of nuclear weapons. To provide an incentive for other countries to not acquire their own nuclear weapons, longstanding US policy is that “the United States will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear non-proliferation obligations,” as re-affirmed in the Trump administration’s 2018 Nuclear Posture Review. The Obama administration’s 2010 Nuclear Posture Review clarifies that this applies even if those states use chemical or biological weapons. Iran does not possess nuclear weapons and, according to the IAEA and UN Security Council, is in compliance with the NPT. Though the Trump administration may have a different interpretation of Iran’s ‘nonproliferation obligations,’ NPT compliance has been the generally accepted standard in the past. Under this standard, using nuclear weapons against Iran would be inconsistent with stated US policy.
However, there was a period between 2005 and 2016 when Iran was not in compliance with its NPT obligations. On September 24, 2005, the IAEA found Iran to be in noncompliance with its safeguards agreement. It remained out of compliance until January 16, 2016, which was the Implementation Day for the Iran nuclear deal. During this period, US policy would not have prevented it from using or threatening to use nuclear weapons against Iran, although attacking Iran with nuclear weapons would still have been illegal under the laws of war.
Q: Who would make the decision to use a nuclear weapon against Iran?
A: In the United States, the president has the sole authority to order the use of nuclear weapons. Under normal circumstances he or she would consult with advisors before making a decision of this magnitude, but there is no requirement to do so. However, a presidential order is necessary to use nuclear weapons, but not sufficient. The president is bound by legal restrictions on the use of military force, and the military is bound not to carry out an illegal order.
Q: What would happen if the United States used a nuclear weapon against Iran?
A: If the military carried out a presidential order to attack Iran with a nuclear weapon, in violation of the laws and customs of war, it would have a profound and negative effect on US security and legitimacy. The United States would become an international pariah, condemned and likely sanctioned.
US nuclear use would break a 75-year-old “nuclear taboo,” setting a dangerous precedent that would increase the likelihood that nuclear weapons would be used again in the future. The NPT—the centerpiece of the international nuclear nonproliferation regime—would likely collapse, as the treaty’s non-nuclear weapon states would no longer have any reason to believe the United States’s long-standing assurances that it would not target them with nuclear weapons as long as they remained in compliance with their treaty obligations. Iran—unless it was under US control—would almost certainly be one of the first to respond by developing its own nuclear weapons as quickly and clandestinely as possible, to ensure that it could threaten retaliation to prevent any future attacks. Without perfect intelligence and ongoing military intervention, the United States would have no way to prevent this from happening.
The collapse of the nonproliferation regime would hurt the United States more than any other country. The United States has the strongest conventional military in the world, and nuclear weapons are the only serious threat to the US homeland. Increasing the chances that other states might also choose to use nuclear weapons is diametrically opposed to US security interests.
A US nuclear attack would further destabilize an already volatile region, possibly drawing other nuclear powers into the conflict and creating the danger of further nuclear escalation. Even if the conflict remained limited to the United States and Iran, it would empower hardliners in other countries and increase the threat of terrorist attacks against the United States and its troops worldwide. And at least some of the 46 nations and foreign territories that are host to more than 500 US military installations would demand their removal.
The physical effects on Iran and beyond would depend on many factors, including the explosive power (or “yield”) of the weapon(s), the altitude at which they were exploded (e.g., a ground explosion will generate more radioactive dust—or fallout), the wind pattern, and characteristics of the target (e.g., many people would die by fire in cities with buildings that are made of highly flammable materials).
The current US nuclear arsenal contains a variety of weapons—bombs, warheads carried by air-launched cruise missiles, and warheads carried by ballistic missiles in silos and on submarines—with 13 different yield options. (The yield is usually expressed in kilotons (kT) of TNT.)
At the low end of the spectrum, the United States has bombs that are only 2% as powerful as the bomb it dropped on Hiroshima in World War II, which essentially leveled that city, killing nearly 200,000 people. (The Hiroshima bomb had a yield of 15 kT; the current bomb 0.3 kT.) The most powerful deployed weapon is carried by ballistic missiles on submarines; it has a yield of 455 kT—about 25 times that of the Hiroshima bomb.
Using the least powerful bomb to target a site in the desert far from towns and cities could result in relatively few deaths and little destruction. However, even such an attack could spread radiation throughout the Middle East.
A study on the consequences of possible nuclear conflict between Iran and Israel concluded that Iranian cities are especially vulnerable to the effects of nuclear attack due to population density, building construction, and geography. One 15 kT weapon detonated on the city of Karaj (pop. 1.1 million) would kill 160,000 and injure another 130,000 people. Using a 250 kT weapon would kill 740,000 and injure 210,000 people, whereas a 500 kT weapon would kill 890,000 and injure 160,000.
Thus, the US use of even a single nuclear weapon on an Iranian city could mean death and grievous injuries to hundreds of thousands of civilians. Many not killed in the initial blast would be killed by the shockwaves, fires, or radiation that would follow. Of course, such an attack would also have grave longer-term consequences for Iran’s economy and the well-being of its citizens.
Editor’s note: This post was erroneously attributed to Lisbeth Gronlund on first publication.
Support from UCS members make work like this possible. Will you join us? Help UCS advance independent science for a healthy environment and a safer world.