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.