The Lessons of 9/11 for October 7

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October 7 is being called Israel’s 9/11. The comparison is apt for the lessons that can be learned as to the legality of launching a ground offensive to respond to terrorism. The United States and the United Kingdom invaded Afghanistan in response to 9/11. Israel has announced it will undertake a full-scale ground invasion of Gaza in the wake of Hamas’ brutal terrorism launched on October 7. The U.S. fought for 20 years in Afghanistan only to be defeated. This failure and the extraordinary ongoing suffering associated with it could well have been avoided through close compliance with authentic international law.

Other analogies have been made for October 7. President Biden has compared Hamas to both ISIS and Russia. The American journalist James Risen compares the events of that day to a “prison riot”. The list goes on, but for purposes of this post, 9/11 is the prior case that best guides legal analysis, which begins, as it did for the invasion of Afghanistan, with the United Nations Charter. Articles 2(4), 39-42, and 51 apply, as well as the general principles of necessity and proportionality. Law on the conduct of armed conflict has already been widely analyzed in connection to the Israel-Hamas crisis, for example, here and here.

Charter Provisions

All analysis of the use of military force begins with the ancient jus cogens prohibition codified in Article 2(4). As jus cogens, the ban arguably applies to all significant uses of military force —whether by states or non-state actors in any circumstances. (For a development of this argument, see Mary Ellen O’Connell, The Art of Law in the International Community (2019).) Article 2(4) as a treaty rule applies to the interstate use of force by UN members. The Charter includes two express limits on Article 2(4). The UN Security Council may authorize force under Articles 39-42, and states may resort to force in individual and collective self-defense in Article 51. The third possible limit is found in general international law and involves the consent or invitation of a government to use force on the government’s territory. Intervention by invitation is irrelevant to either 9/11, October 7, or authorization by the UN Security Council.

That leaves Article 51, the provision invoked by the U.S. when it reported to the Security Council its justification for using force against Afghanistan. Prior to 9/11, international lawyers generally agreed that even significant acts of terrorism were criminal acts that did not implicate Article 2(4), let alone trigger Article 51. Then, in reaction to 9/11, the UN Security Council incorporated terms from Article 51 –“inherent right of self-defense”– in a preambular part of Resolution 1373 condemning the terrorist attacks in the U.S. The Council made no further findings as to how a military response to terrorism can possibly meet the legal conditions for lawful resort to force in self-defense.

Article 51 permits a state to carry out military operations on the territory of a state responsible for an armed attack on a defending state. The armed attack requirement is the “sine qua non” requirement of Article 51. (Nicaragua, para. 237) The law of state responsibility requires that all coercive action taken to enforce rights must be aimed at a party responsible for the wrong. The U.S. never provided real proof that the Taliban, Afghanistan’s de facto government, was legally responsible for 9/11. Al Qaeda was the responsible party, a non-state actor operating from bases in Afghanistan and Germany. Article 51 does not apply to armed attacks by non-state actors. The negotiating history of Article 51 is clear on this point, which the ICJ confirmed in the Wall Opinion. The Court said that Article 51 does not apply to territories under the control of a state that is attempting to justify the use of force by invoking it. Israel maintains that it has not occupied Gaza since withdrawing troops from inside the enclave in 2005. However, as shown again in the post-October 7 period, Israel has never relinquished control of Gaza’s borders. Defensive measures commensurate with the human rights protections owed by a government to people under its control are, of course, permissible. Israel also has effective defenses like the Iron Dome shield system that can be deployed on its own territory.

Even just attempting for purposes of this analysis to apply Article 51 to October 7 faces insurmountable hurdles. The triggering attacks must be attributed to a state. Gaza is not a state. Hamas has some governance authority in Gaza, but Hamas is not a government comparable even to the Taliban in Afghanistan. Israeli Prime Minister Netanyahu is widely considered responsible for raising the status of Hamas in order to weaken the Palestinian Authority’s governance of the combined West Bank and Gaza. Charging Hamas militants with the crime of international terrorism is easy; assigning state responsibility to Gaza, Palestine, or Palestinians collectively to justify an invasion is not.

Necessity and Proportionality

Even where Article 51 permits resort to force by a defending state, the general principles of necessity and proportionality must be met. Necessity requires that force be used only as a last resort and only when there is a reasonable chance of success. (Daniel Statman, ‘On the Success Condition for Legitimate Self-Defense’, 118 Ethics 659, 660 (2008).) The principle of proportionality requires that the force used be commensurate to the injury. Even if the U.S. had had the right to invade Afghanistan under Article 51, the invasion did not meet the requirements of necessity or proportionality.

First, invading Afghanistan was no last resort. The Taliban were open to cooperating with the U.S., an option that stood a far better chance of capturing and prosecuting the leaders of Al Qaeda than an invasion. According to Lawrence Wright, The Looming Tower: Al-Qaeda and the Road to 9/11 (2006), the Taliban planned to expel Al Qaeda after 9/11. An invasion is not a last resort for Israel either. Alternatives to force exist that are all but guaranteed to succeed in ending Palestinian violence against Israel. For example, compliance with UN Security Council Resolution 242 to withdraw from territory captured in the 1967 Six-Day War. If Israelis do not have the political will to opt for alternatives that will work, the obligation of last resort is not met and conditions to resort to force under Article 51 are not met.

A full-scale invasion has no reasonable chance of success in ending violence against Israel. Article 51 is designed for a state to halt and repel occurring armed attacks. There is little, if any, evidence that Al Qaeda, let alone the Taliban, planned more attacks against the U.S. in the days or even weeks after 9/11. Rockets continue to hit Israel from Gaza; a full-scale invasion may stop them in the short run but not over time. President Biden urged Israeli leaders to learn from the mistakes the U.S. made after 9/11. U.S. defense officials consider those lessons to include the risk that Israel could make matters worse. Hamas may well be destroyed but at the risk of “creating more militants”. In the case of Gaza, there is evidence beyond Afghanistan and America’s “war on terror” that an invasion will not succeed. Israel has used force persistently throughout Palestinian territory for decades. This force is almost always followed by more terrorism and violence carried out by Palestinians. Israel even launched a major ground offensive in Gaza in 2009, only to conclude it had failed, trying again in 2014. October 7 was the worst case of violence against Israelis since the founding of Israel. The foreseeable outcome of yet another invasion is another failure.

Even where a military invasion meets Article 51 and necessity, the use of force must also be proportionate to the injury. Proportionality is typically mentioned in combination with necessity. It is distinctive, however. Like necessity, it is mentioned in at least four ICJ judgments but never in detail. Still, in the case of 9/11 and the invasion of Afghanistan, several governments, including Afghanistan’s interim government, which the U.S. helped install and kept in power, criticized the continuance of bombing after the Taliban fell in December 2001. Continuing to use that amount and type of force was disproportionate. The Afghanistan invasion in general can hardly be characterized as anything else.

The overriding lesson of 9/11 for October 7 is that ground invasions following terrorism cannot meet the conditions of lawful self-defense. International law provides effective responses to terrorism that have led in the past to peace. They can do so again.

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Brad R. Roth says

October 28, 2023

Thanks, Mary Ellen, for your consistent championing of peace. I would, however, seek to avoid reliance on the ICJ's formalistic interpretation of Art. 51. The attack was at a level of severity previously (e.g., in 1974, the time of GA Res. 3314) expected only from states or their proxies, and emanated from territory that, although remaining subject to occupation as a de jure matter, is not under the occupying state's military control. I would prefer to rely on jus in bello principles, given the improbability of being able to avoid civilian losses "excessive in relation to the concrete and direct military advantage anticipated."

Noemi Gal-Or says

October 28, 2023

In 2008, at the Rio de Janeiro Seventy Third ILA Conference, you and I had a brief exchange just before we entered a session discussing R2P. You were adamant that R2P had by then been established as a new IL norm. I was sceptical. Both of us have publications on R2P. I am therefore surprised that at the end of your piece above, when referring in general to the effective responses to terrorism, no specific response is suggested, especially, R2P isn’t mentioned. R2P is certainly worth consideration as an option to free the Palestinian people in Gaza (and possibly in the West Bank where Hamas’ power grab is imminent) from the yoke of Hamas and consequently also pave the way to the reduction of terrorism.

Shira Sabbagh says

October 28, 2023

The author invites us to believe that:

1. Hamas is not really the government of Gaza. (It is.)

2. Israeli withdrawal behind the 1967 border is all but certain to produce peace. (It didn't.)

3. Proportionality under IHL is with respect to one's own losses and not military necessity. (It isn't.)

No wonder Israelis don't take the professions of "international law experts" seriously.

Mary Ellen O'Connell says

October 28, 2023

Dear Brad,

Thank you for your kind words and your engagement with the post. I deeply value your knowledge and insight across international law, but on this rare occasion must disagree with you. The Wall opinion must be cited. It is a decision of the ICJ and is on point. The opinion's formalism is a strength--an antidote to the sort of "realism" that has led to the current crisis of respect for Article 2(4).

You object to the formalism in the Wall case, I believe, because you see Gaza as not under Israeli military control as of October 7. The Wall case and Congo (para. 147) indicate otherwise.

Even if Article 51 applied, however, there is another obstacle about which I believe we do agree--the necessity principle of the jus ad bellum, which restricts force to a last resort and where there is a reasonable chance of success.

Finally, we cannot ignore the law on resort to force. It determines whether a state responding to high levels of violence must do so at the higher human rights standards respecting the right to life or only the lower standards of IHL. As I write, there is fighting between organized armed groups in Gaza and Israeli forces. IHL now applies.

Nevertheless, whether Article 51 applies or not is still critical. For example, Article 51 determines the legality of providing military assistance. Ukraine has the right to receive military assistance in collective self-defense under Article 51. Russia does not. No state may lawfully provide military assistance of any kind to Russia.


Mary Ellen

Mary Ellen O'Connell says

October 28, 2023

Dear Noemi,

Every person must have their human rights respected--something only possible with good governance.

But, while I was in Rio, I have never been a proponent of "R2P" to the extent it relies on using military force to attempt to improve human rights. To do so is neither lawful or wise. I have included this critique in almost every publication dealing with the use of force, starting soon after the Kosovo Crisis. Here is just one specific example: "Responsibility to Peace: A Critique of R2P", 4 J. of Intervention and Statebuilding 39 (2010).

I can only think it was someone else on your panel?


Mary Ellen

Raphaël van Steenberghe says

October 29, 2023

Many thanks for this interesting post. In my opinion, however, the issue is even more complex. The first question to examine is whether the argument of self-defense to justify the Israeli military response to the attacks by the Hamas is relevant at all – although referred to by several states (see e.g. the US, S/PV.9439, at 2; UK, ibid., at 4; Malta, ibid., at 5; Ecuador, S/PV.9442, at 9) – and, if not, which regulation applies. The answer depends upon the view adopted on several controversial issues including:
i) whether the State of Palestine is treated as a State under international law (self-defense is only meaningful when invoked to justify a use of force against or in the territory of another state, as it is an exception to the prohibition on interstate use of force);
ii) whether the Gaza Strip is still considered as an occupied territory (as emphasized by the ICJ in the Wall case as well as by states [see e.g. Jordan, S/PV.9439, at 12], self-defense is meaningless when the armed attack comes from a territory occupied by the state victim of that attack);
iii) whether the occupation of the State of Palestine by Israel (including at least West Bank and East Jerusalem) is/remains legal under jus ad bellum, even though some of its practices as an occupying power, such as the practice of settlements, are clearly unlawful (it is questionable whether a new argument based on self-defense is meaningful if the initial jus ad bellum justification for the maintenance of the occupation, based on security concerns, might still operate).
In addition, regarding the conditions for the application of the argument of self-defense, it cannot be asserted so straightforwardly, in my view, that ‘Article 51 does not apply to armed attacks by non-state actors.’ This is a debated issue and there is strong state practice against such a position.

Hartmann says

October 29, 2023

Prof. O'Connell. I can only say "thank you so much". I have been struggling with this subject for days, and you put it all in a clear, concise, way such that even non-legal people like me could understand. Thanks to this article, I now have enough to give some explanations to friends who are asking me questions about the frustrating, sad, and desperate situation in this unending saga.

Miri Sharon says

October 29, 2023

I think the attempt to compare the ongoing war in Israel to the 9/11 attack has no merit in the facts or the context.
1. first, the attack on civilians is still ongoing, every day hundreds of rockets are still fired towards civilians in cities in Israel, killing indiscriminately. This has been going on for 3 weeks now non-stop. In other terms, this is an ongoing attack.
2. Afghanistan is 12,000 km away from NY. Gaza is 5 km away from the cities attacked. There is an immediate need to stop the attack.

Finally, the author suggests international law has good suggestions for responding to terrorism, but lists none. If this implies Chapter 7 action, we are all aware the Security Council is currently paralyzed.

If authors are eager to write blogs to learn from History at this time, I suggest looking at the war in the former Yugoslavia. Read the judgments of the ICTY. The brutality of rapes and torture committed by Hamas, motivated by their goal to eliminate Jews, remind us more than anything else of those horrible days in human history.

Mary Ellen O'Connell says

October 29, 2023

Dear Shira, Raphaël, Hartmann, & Miri,

Thank you all for your comments. I would like to discuss in more detail but given the limits of the format will restrict myself to three points:

1. Building respect for international law, especially on the use of military force, is an urgent necessity. Decline in respect, indicated by Shira, tracks the trend toward lawyers shaping their interpretations to meet officials’ preferred policies. The result is there is no law to respect—there is nothing to conform to or comply with. The law is just adapted to meet the policy and officials claim they value the rule of law. For details of the phenomenon, see, SAMUEL MOYN, HUMANE: HOW THE UNITED STATES ABANDONED PEACE AND REINVENTED WAR (2021); REBECCA SANDERS, PLAUSIBLE LEGALITY: LEGAL CULTURE AND POLITICAL IMPERATIVE IN THE GLOBAL WAR ON TERROR (2018); Victor Kattan, Furthering the “War on Terrorism” through International Law: How the United States and the United Kingdom Resurrected the Bush Doctrine on Using Preventive Military Force to Combat Terrorism, 5 J. USE FORCE & INT’L L. 97 (2018), and Jutta Brunée & Stephen J. Toope, Self-Defence Against Non-State Actors: Are Powerful States Willing but Unable to Change International Law?, 67 INT’L & COMP. L.Q. 263 (2017).

The remedy is fidelity to formal law.

2. The control issue is key to the ICJ decisions, as I mention in the piece and in response to Brad Roth, not whether Hamas is a government or even whether Palestine is a state. On whether NSAs can breach Article 51, the Charter negotiating history does not support it; the ICJ has only hinted at the possibility when an NSA is in control of territory, see Congo v. Uganda, para. 147. In my view the great weight of global scholarly opinion does not find “strong state practice” in favor of interpreting Article 51 in this way. The argument for doing so originates in a few states that are actually using military force against non-state actors on foreign state territory, so it serves their interests but not that of law compliance. Finally, even if this state practice was joined with enough opinio juris to create a new rule of customary international law or a reinterpretation of Article 51, it would derogate from Article 2(4), a jus cogens norm from which no derogation is permitted. For details on all of these points, see, Mary Ellen O’Connell, Forever Air Wars and the Lawful Purpose of Self-Defence, 9 J. Use of Force and Int’l L. 33 (2021), DOI: 10.1080/20531702.2022.2029020.

3. Comparing 9/11 and October 10 is appropriate, despite the obvious differences, because governments are shaping their legal positions around the comparison. Raphaël provides cites in his comment. Also, 9/11 was a turning point on the very issue of whether a state may use major military force permitted under Article 51 in response to a terrorist attack. Prior to 9/11, terrorism, even persist, highly violent terrorism of the type the Irish Republican Army and other Northern Irish groups perpetrated was treated under international law as crime. It does not have the characteristics regulated under the jus ad bellum or the jus in bello. The British used military forces to respond to high levels of armed violence in Northern Ireland. When policy changed to follow international human rights law more closely, they created the opening that led to the Good Friday accords. Mexico has had to use military forces to respond to the extraordinary power of organized crime groups, which carry out governance in some places. Mexican forces must follow international human rights law (IHRL). One major difference between IHL and IHRL is that IHRL does not permit even civilian casualties, even if unintentional and proportionate to the legitimate goal of law enforcement. Israel has all the capacity to respond to the violent terrorism, including rocket attacks, following IHRL. Social science data supports following IHRL for peace and security. By contrast, another major ground invasion has no reasonable chance of success.

UNSC Res. 1373 opened the door to trying to respond to terrorism with military force. But, as I argue, it was never a complete authorization and could not be, given the differences between NSA terrorism and self-defense to an armed attack by a state when necessary and proportionate.


Mary Ellen

Juna says

October 30, 2023

Thank you for this insightful article. I was surprised to see that the article directly jumped to the problems under Art. 51 UNC. I think that we even need to take one step back: Israel would only need to seek to rely on Art. 51 if its actions violated Art. 2(4) UNC in the first place (A justification only becomes necessary once the main provision has been violated). Could you maybe provide some clarification on that? How do Israel's actions in Gaza violate the "territorial integrity of another state" as purported by Art. 2(4) UNC? If this cannot be shown, Art. 51, in my eyes, does not apply as there is no violation of Art. 2(4) UNC but only potentially of IHL.

Mary Ellen O'Connell says

November 1, 2023

Thanks, Juna, for the kind words and the question. Any major use of military force by a state that is not justified by UN Charter Articles 51 or 39-42 or by an invitation by a government in effective control violates Article 2(4). The drafters intended it to be “an absolute all-inclusive prohibition; the phrase ‘or in any other manner’ was designed to ensure that there should be no loopholes.”

Article 2(3) mandates using peaceful means to settle disputes.

Mary Ellen

John Dehn says

November 1, 2023

Respectfully, Mary Ellen, it seems a bit disingenuous to claim that Gaza is not a state for purposes of Art. 51, but that it is protected by Art. 2(4), which prohibits the use of force against "any state" or in other ways "inconsistent with the purposes" of the Charter. Addressing a threat that has not only engaged in a recent and horrific terrorist attack, but that also regularly engages in indiscriminate attacks against the Israeli population, does not seem to me to be inconsistent with the purposes of the Charter. If Gaza is not a state, how exactly does the "jus cogens" norm of Art. 2(4) apply?

Regarding jus ad bellum proportionality, there is clearly a disconnect between state practice and academic commentary. States seem to believe that they may eliminate threats that are likely to recur. This has origins in pre-Charter (and post-Charter) practice. Moreover, Art. 51 provides that a state's inherent right of self-defense continues until the Security Council "has taken measures necessary to maintain international peace and security." Because that almost never happens, states are left to address these threats themselves. If they choose to try to eliminate a threat, that seems to be consistent with the Charter and the inherent right of self-defense that existed pre-Charter, especially with respect to non-state actors.

I do not disagree with your policy positions but do agree with your legal analysis in all of its particulars.

John Dehn says

November 1, 2023

That should have said I agree with your policy positions but I disagree with your legal analysis. (Sorry - I haven't commented on a blog in quite some time!)

Nicolas Boeglin says

November 2, 2023

Dear Profesor O´Connell

Many thanks for your very valuable post.

It is quite astonishnig to see the same errors been committed, with the (wrong) idea that a military response is a good solution in these cases.

"Comparaison n´est pas raison" we say in French, and there are some differences between theses two events.

However, the "comparison" between 9/11 an 7/10 can also be transferred to UN main organs´s reactions: as you remember, UNSC resolution was adopted 1368 unanimously on sept 12, 2001.

In the present case, UNSC has failed 4 times to adopt a resolution, and it is the UNGA that finally has been able to adopt a resolution, last last Oct. 27.

This detail can allow some of us to say that the two events are not excalty similar for a comparison.

By the way, if you or our Canadian colleagues at EJIL-talk read me, please feel free to explain us the extreme solitude of Canada in order to obtain an amendment paragraph (Document available here

finally rejected just before the adoption of UNGA resolution (adopted with 120 votes in favour, 14 agaisnt and 45 abstentions).

It is quite bizarre to see one single State trying to present an amendemnet on such sensitive issue and wording.

I have tried to analyse this strange move in this modest note (in French, sorry):

Yours sincerely

Nicolas Boeglin

Larry Csillag says

November 8, 2023

1. It is difficult to reconcile the ICLJ analysis of force with it's failure to address the legal and moral aspects of multiple continuing crimes against humanity, the holding of civilian (and other)hostages.
It should be hard to ask for respect for the law, when the legal analysis of specific details of the use of force come before and to the exclusion of crimes against humanity.

Moral and legal regard for "peace" before hostages miss the point and purpose of supporting peace and condemn the analysis as supportive of the crime. A demand for the rule of law to support a criminal order of society is what defines the narco-state, not an international law journal.

"Every person must have their human rights respected--something only possible with good governance" is an empty phrase while hostages are being held and the animus is to dilute the military force aimed at freeing those hostages. Arguments to the effect the military force lacks proportionality or has subsidiary or additional goals only serve to make the analysis part of an accessory after the fact. The appropriate legal framework is a narco-terrorist takeover, or perhaps a Guinea-Bissau narco state, not a state response to an attack by another state, an article 51 centered analysis. The scale of the operation simply changes the means of the response from a police operation to a military one.

2. Is Israel responding to a "previous" attack, or using its best military approach to defend against an ongoing attack? It would be disingenuous to address this point with the intentions of Hamas. Does article 51 now purport to control the military weapons and tactics used to defend and defeat a continuing attack? Miri Sharon (above) accurately but not quite emphatically enough makes that point. The 500 or so dead at the hand of the continuing attack of Hamas allied forces at al-Ahli Arab Hospital underline that Gaza itself has been held hostage for years without any international legal opprobium. The appropriate hostage count is a million or more.

3. Are the Palestinians continuing to hold these hostages or is Hamas?

There are many more issues that remain unadressed by the facile article 51 approach. Perhaps the ICLJ can address them in the future.