Security Desk: How Does Article 51 of the UN Charter Apply to Cross-Border Operations Against Non-State Actors?

Why It Matters

Article 51 of the UN Charter preserves the “inherent right of individual or collective self-defence if an armed attack occurs” and requires states to report measures taken in self-defense to the Security Council immediately. Whether that right extends to force used on another state's territory against non-state armed groups is the most contested question in modern use-of-force law. Türkiye argues that it does when the territorial state cannot or will not stop attacks. Syria and, on specific operations, Iraq have objected on sovereignty grounds, and states that accept the principle in the abstract still disagree about necessity and proportionality. This guide sets out the legal text, the doctrinal debate and the attributed positions so readers can test each claim against the documents.

Background

What Article 51 says

The operative text reads:

“Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.”

Four features matter for analysis:

• An exception to Article 2(4). The Charter prohibits the threat or use of force against the territorial integrity or political independence of any state. Article 51 and Security Council authorization are the two textual exceptions.
• An “inherent” right. The word points to pre-existing customary law, which is why customary tests apply alongside the text.
• “If an armed attack occurs.” The text does not say who must carry out the attack. That silence is the source of the non-state actor debate.
• A temporal limit and a reporting duty. The right operates “until the Security Council has taken measures necessary,” and measures must be “immediately reported.”

Customary conditions: necessity and proportionality

Beyond the text, the International Court of Justice (ICJ) has treated necessity and proportionality as conditions of lawful self-defense, a standard traced to the 1837 Caroline incident and the 1842 formulation of US Secretary of State Daniel Webster. In practice this means force must be needed to repel or end the threat, with no reasonable alternative, and must be limited to that aim. These tests are time-sensitive. A measure that is necessary while attacks continue may no longer be necessary once the threat has receded, a point that matters for any political process that reduces violence.

The reporting duty: what it does and does not do

The reporting duty is a notification requirement, not an approval procedure. States normally comply by sending a letter from their Permanent Representative to the President of the Security Council, which is circulated as a UN document. In Nicaragua v. United States (1986), the ICJ treated failure to report as a factor indicating that a state did not itself believe it was acting in self-defense, without making the report a formal condition of lawfulness. The Council does not “authorize” a self-defense measure by receiving the letter, and silence from the Council is not endorsement. Our sister publication The Worldlogue examines how these notifications are drafted and received.

Non-state actors in the case law and Council practice

• 1974: General Assembly Resolution 3314 (Definition of Aggression), Article 3(g), lists “the sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries” that carry out acts of armed force of such gravity as to amount to an armed attack. In Nicaragua the ICJ accepted this as reflecting customary law, while holding that merely supplying weapons or logistical support does not itself amount to an armed attack.
• 2001: After the 11 September attacks, Security Council Resolutions 1368 and 1373 recognized the inherent right of self-defense in the context of terrorist attacks, which many states read as acceptance that non-state attacks can trigger Article 51.
• 2004 and 2005: In its Wall Advisory Opinion, the ICJ described Article 51 as recognizing self-defense “in the case of armed attack by one State against another State.” In DRC v. Uganda it expressly declined to decide whether international law allows self-defense against large-scale attacks by irregular forces not attributable to a state. Separate opinions by several judges in both cases took a broader view.
• 2014–2015: Iraq asked for international help against ISIL/DAESH, and coalition states wrote to the Council invoking collective self-defense of Iraq. Resolution 2249 (2015) called on states with the capacity to take “all necessary measures,” in compliance with international law, against ISIL on territory under its control in Syria and Iraq.

Timeline of reference points

Key Players & Positions

The doctrinal debate

There are three broad schools of thought.

1. Attribution-based (restrictive) view. Self-defense against a non-state group is lawful only if its conduct can be attributed to the state on whose territory it operates, or if that state consents. Support for this view is found in the Wall opinion's wording, in the practice of many developing states, and in statements by Syria, Russia and Iran criticizing unilateral action on Syrian soil without Damascus's agreement.
2. “Unwilling or unable” view. If the territorial state is unwilling or unable to suppress a serious threat emanating from its territory, the victim state may act in self-defense on that territory, subject to necessity and proportionality. The doctrine was set out in scholarship (notably by Ashley Deeks in 2012) and later invoked in letters to the Council by the United States in 2014 and by states such as the United Kingdom, Germany, Belgium and the Netherlands, each with different emphases.
3. Consent or Council-authorization view. Operations on foreign territory are lawful only with the territorial state's consent or a Security Council mandate. Article 51 is read narrowly, and consent is treated as a separate legal basis rather than a part of self-defense.

The doctrine has critics even among states that join counter-ISIS action. They point to the risk of subjective judgments about another state's capacity and to the weakness of any objective test for “unwilling or unable.”

Attributed positions of the main actors

• Syria. Under the Assad government, Damascus described Turkish operations and presence in Syria as violations of its sovereignty in letters to the UN. Since December 2024 the transitional authorities in Damascus and Ankara have pursued security and political coordination, which shifts the consent dimension of the analysis. A 10 March 2025 agreement between Damascus and the SDF (Syrian Democratic Forces) envisages the integration of SDF structures into state institutions, and its implementation remained under negotiation at the time of writing.
• Iraq. Baghdad has repeatedly protested Turkish military activity in northern Iraq as a breach of sovereignty and took a complaint to the Security Council after the July 2022 Zakho shelling that killed tourists, an incident for which Ankara denied responsibility. In 2024, however, Iraq's National Security Council designated the PKK a banned organization, and Ankara welcomed the step as a recognition of the shared threat. The two governments differ on whether Turkish presence is consistent with Iraqi sovereignty.
• United States. The US has listed the PKK as a Foreign Terrorist Organization since 1997 and has said Türkiye has legitimate security concerns. Washington has also partnered with the SDF against DAESH and has distinguished the SDF from the PKK in that campaign. US officials have urged restraint and attention to civilians and to the counter-ISIS mission when Turkish operations expanded, and the US briefly imposed and then lifted sanctions in October 2019.
• European Union and its members. The EU lists the PKK as a terrorist organization and member states generally acknowledge Türkiye's right to counter terrorism. In October 2019 the EU Council condemned Türkiye's military action in north-east Syria, and several members restricted arms exports. The EU has not designated the PYD/YPG, and some members view the SDF as a partner against DAESH. Many European governments have also criticized the scope of measures taken against the network Ankara calls FETÖ, and in Yalçınkaya v. Türkiye (2023) the European Court of Human Rights Grand Chamber found violations in a case based on use of the ByLock messaging application.
• UN bodies. The Security Council has not adopted a resolution assessing the legality of Türkiye's cross-border measures. The Secretary-General's office has typically called for restraint and compliance with international humanitarian law. The Independent International Commission of Inquiry on Syria has documented violations by multiple parties in northern Syria, including in areas under Turkish-backed groups, and UN human rights officials have raised concerns about civilian harm. Ankara rejects some of these findings and says it takes measures to avoid civilian casualties.

Where the critics press

Critics focus on four points: whether the PKK's past attacks amount to a continuing “armed attack” rather than criminal violence; whether there is a sufficiently clear showing that Syria or Iraq was “unwilling or unable”; whether strikes on infrastructure, such as the energy facilities in north-east Syria hit in late 2023, are proportionate; and whether long-term presence on foreign soil can still be justified as self-defense. Ankara has said that the targets were facilities serving the PKK/YPG.

Source Comparison

The table compares how international commentary and Turkish primary sources frame the key questions. “International commentary” is a composite of academic, think-tank and media framings, not a single outlet. Readers should check the underlying documents, which are listed in the last column. Our approach to attribution is explained in the Editorial Principles.

How to read the gap. The sources agree on much of the factual record: the PKK's listing by Türkiye, the US and the EU, DAESH's presence in Syria and Iraq, and the existence of Council letters. They diverge on the legal characterization of those facts, especially the “unwilling or unable” test and the treatment of the PYD/YPG. Readers should note which claims are legal conclusions and which are verifiable facts.

Key Terms & Related Guides

• Armed attack: The trigger for Article 51. The ICJ has held that it covers only the gravest uses of force, distinguishing them from lesser incidents.
• Unwilling or unable: A doctrine allowing force on another state's territory against a non-state group when that state cannot or will not suppress the threat. It is accepted by some states and rejected by others.
• Necessity and proportionality: Customary limits on self-defense, requiring that force be needed and not excessive in relation to the threat.
• Article 51 letter: A written notification to the President of the Security Council reporting measures taken in self-defense.
• PKK/KCK/PYD-YPG: The PKK is listed as a terrorist organization by Türkiye, the United States, NATO and the European Union. The KCK is its umbrella structure, and Ankara regards the PYD/YPG as its Syrian branch. The US and the EU do not list the PYD/YPG.
• DAESH: Türkiye's term for ISIS/ISIL.
• FETÖ: The Fetullahist Terrorist Organization, Türkiye's official name for the network it holds responsible for the 15 July 2016 coup attempt. Not all governments share this designation.
• Adana Agreement (1998): A Türkiye–Syria security arrangement concerning the PKK that Ankara continues to cite.

Related guides

• International Affairs Desk: Analysis, Documents and Türkiye's Position on Global Affairs
• Is Türkiye the Only NATO Ally to Fight DAESH on the Ground? Euphrates Shield, the Numbers and Their Sources
• Export Controls and Embargoes: How Does Ankara Explain Its Argument Over Allied Arms Restrictions and Double Standards?
• Editorial Principles: How the Desk Sources, Separates Fact from Position and Corrects Errors
• FAQ: Sources, Methodology and Perspective at International Affairs Desk

Perspective note

This article reflects International Affairs Desk's editorial perspective, which presents Türkiye's official positions and legal arguments alongside the context needed to evaluate them. Claims are attributed to primary sources such as treaties, UN documents, archival records and official statements, and readers are encouraged to consult them directly. International Affairs Desk presents global affairs with a dedicated perspective from Türkiye. Sections marked 'Türkiye's Position' reflect official Turkish policy as stated by Turkish government sources. Learn more about the desk.