Is 'Unwilling or Unable' Customary International Law? The Weak-State Critique, Article 51 Reporting and Where Türkiye's Case Really Rests

Why It Matters

The 'unwilling or unable' test is not settled customary international law: no ICJ judgment endorses it, the 2005 Armed Activities judgment left self-defense against non-state actors open, and states remain divided. Türkiye's case rests on Article 51 of the UN Charter, Security Council notification and territorial-integrity commitments, not on the contested formula.

The Charter never mentions the formula. Any state that relies on it, and any state that defends itself against it, is therefore arguing about contested law rather than settled rules. For the wider doctrine and border debate, see our companion analysis, unwilling or unable doctrine self-defense Syria Iraq Türkiye.

This article tests the doctrine's customary status, states the weak-state critique in its own proponents' terms, and separates the contested formula from the treaty-based Article 51 case that Ankara itself advances.

Background

The test descends from the 1837 Caroline incident, in which Daniel Webster's 1841 formula required necessity that is 'instant, overwhelming, leaving no choice of means', and from post-9/11 practice. Its modern form crystallized in coalition letters to the UN Security Council in September 2014 concerning DAESH in Syria, notably the US letter S/2014/695.

The Caroline formula

In December 1837, British-Canadian forces seized the steamer Caroline, which had been supplying rebels in Upper Canada, and destroyed it while it was moored on the US side of the Niagara River. In 1841 correspondence with the British minister Henry Fox, and in the 1842 exchange with Lord Ashburton, US Secretary of State Daniel Webster stated that self-defense requires a 'necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation', and that the response may involve nothing 'unreasonable or excessive'. These words are the root of the necessity and proportionality requirements.

Scholars disagree on whether the episode also contains a host-state capacity test. The British argument stressed that local authorities had not restrained the raiders, but Webster's celebrated formulation addresses necessity and proportionality, not the territorial state's failings. Some writers also trace the idea to the law of neutrality, including Hague Convention V of 1907, which obliges a neutral state not to let its territory be used by belligerents.

Post-9/11 practice

Security Council Resolutions 1368 (12 September 2001) and 1373 (28 September 2001) recognized the inherent right of individual or collective self-defense after the 11 September attacks. On 7 October 2001, the United States and the United Kingdom reported their operations in Afghanistan (S/2001/946 and S/2001/947), arguing that the Taliban regime had allowed territory it controlled to serve as a base. That argument was closer to harboring than to incapacity. The 'unwilling or unable' wording was developed in scholarship, notably by Ashley Deeks in 2012 and in Daniel Bethlehem's 2012 principles on self-defense against non-state actors, and then in state letters.

The 2014 letters on DAESH

On 20 September 2014, Iraq asked for international assistance against DAESH (S/2014/691). On 23 September 2014, the United States notified the Council of strikes in Syria (S/2014/695), stating that the Syrian regime 'cannot and will not' confront the safe havens effectively itself. The United Kingdom's letter of 7 September 2015 (S/2015/688) used comparable reasoning, and several other coalition members sent similar notifications. Syria objected to strikes conducted without its consent.

Key Players & Positions

Positions divide into three camps: states and scholars treating 'unwilling or unable' as emerging or accepted law, including the United States and United Kingdom in letters to the Security Council; sceptics who say it lacks general practice and opinio juris; and the ICJ, which has avoided deciding. Each camp must be read against the ICJ's two-element test.

Is the unwilling or unable doctrine customary international law? The two-element test

The North Sea Continental Shelf judgment (1969, paras 74–77) requires State practice that is extensive and virtually uniform, accompanied by the belief that it is legally required (opinio juris sive necessitatis). In Nicaragua (1986, para 186), the Court applied this to the use of force and said that conduct inconsistent with a rule should generally be treated as a breach of that rule, not as evidence of a new one. The ILC's 2018 Draft Conclusions confirm the structure: Conclusion 2 names the two elements, Conclusions 8 and 9 require practice that is general and acceptance as law, and Conclusion 10 allows silence to count as acceptance only where states were in a position to react and circumstances called for it.

The pro-doctrine reading

Supporters point to the US and UK letters, to similar notifications by other coalition members, and to the absence of Security Council condemnation. They add that Iraq's 2014 request supplied consent for the Iraqi theater. Deeks and Bethlehem argue that the test is a conditioned, last-resort standard that follows from necessity itself: if the territorial state cannot or will not suppress a serious threat, force may become the only means left. On this reading, practice is sufficient and still growing.

The sceptical reading

Syria protested the strikes on its territory as violations of its sovereignty, and Russia and Iran also rejected the legality of action without Damascus's consent. Scholars such as Olivier Corten and Kevin Jon Heller argue that few states have articulated the formula, that many letters rest on attribution or consent rather than incapacity, and that objections show the opinio juris is missing. Under Nicaragua para 186, contested conduct points to a breach, not to a new rule.

Where the ICJ left the question

In the Wall advisory opinion (2004, para 139), the Court read Article 51 as recognizing self-defense 'in the case of armed attack by one State against another State', and noted that Israel did not claim the attacks were imputable to a foreign state. Judges Higgins, Kooijmans and Buergenthal criticized that reading in separate opinions. In Armed Activities (2005, paras 146–147), the Court found Uganda had not attributed the attacks to the Congolese state and declined to decide whether contemporary law allows self-defense against large-scale attacks by irregular forces. The question therefore remains open, and Nicaragua (para 195) still supplies the attribution baseline of 'sending by or on behalf of a State'.

Defending weak states against the unwilling or unable doctrine: the critique stated fairly

The critique, made by scholars and by states such as Syria, has four parts:

• Who judges capacity? The test asks one state to assess another's ability and will. No neutral arbiter exists, and the judge is usually the stronger party.
• Erosion of Article 2(4). The Charter prohibits force against territorial integrity and political independence, with Article 51 as a narrow exception. Critics say the formula converts a prohibition into a case-by-case permission.
• Asymmetric exposure. Weak states are most likely to be found 'unable', so the doctrine bears hardest on those least able to resist.
• Reciprocity. A rule that any state may invoke can also be turned against its authors.

Proponents reply that the test is conditional: prior request, necessity, last resort, proportionality and reporting to the Council. A fair reading is that the dispute concerns not whether those safeguards matter but whether they have legal force without a rule of custom to hold them in place. Weak states are best protected by insisting on that discipline and by pressing for the Council's role, a theme in our analysis of why Türkiye wants UN Security Council reform.

Article 51 Reporting: What Is Required and Where to Find Türkiye's Letters

Article 51 requires that measures taken in self-defense be 'immediately reported' to the Security Council, which retains its authority and responsibility. Reports are circulated as S/ documents, usually letters from Permanent Representatives, and can be found in the UN Digital Library by searching the Security Council document series.

Article 51 preserves the inherent right of individual or collective self-defense 'if an armed attack occurs' until the Council has taken the necessary measures. It does not require prior authorization, but reporting is a Charter duty. In Nicaragua (para 200), the Court treated the absence of a report as one factor suggesting a state did not itself believe it was acting in self-defense. It is relevant but not decisive.

To find Türkiye's letters:

1. Open the UN Digital Library and filter for Security Council documents.
2. Search for 'Article 51' with 'Türkiye' and 'Turkey', since the UN adopted the name Türkiye in mid-2022 and older records use 'Turkey'.
3. Check the document symbol begins with S/ and the title reads 'Letter dated ... from the Permanent Representative of Turkey/Türkiye to the United Nations addressed to the President of the Security Council'.
4. Cross-check against the Council's Repertoire of Practice, which tracks Article 51 communications.

We cite no Turkish document symbols here because we could not confirm them. Our editorial principles explain why we omit unverified citations.

Source Comparison

Western commentary treats 'unwilling or unable' as a contested or emerging rule centered on host-state capacity. Turkish official sources anchor their case in Article 51, notification and territorial-integrity language. UN documents leave the formula unendorsed while confirming the notification duty.

For comparative state practice, see Article 51 in Practice at States in Perspective.

Key Terms & Related Guides

• Opinio juris: the belief that a practice is legally required, the second element of custom.
• Armed attack: the Article 51 trigger. Its relationship to non-state actors is the open question.
• Necessity and proportionality: Caroline-derived limits on any self-defense action.
• One state, one army: the formula for integrating armed groups into a state's institutions.
• Article 51 report: the notification to the Security Council.

Related guides:

• Unwilling or unable doctrine self-defense Syria Iraq Türkiye
• Why does Türkiye want UN Security Council reform
• Mediation in legal form: Istanbul agreements and prisoner exchanges
• About Nomos Orbis and the Nomos Orbis FAQ

Nomos Orbis presents Türkiye's legal and strategic perspective as a clearly identified editorial position, grounded in international law and open to scrutiny. Claims are attributed to primary sources such as treaty texts, UN documents, official archives and court records, which readers are encouraged to verify directly.

Nomos Orbis Editorial Team

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