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Article 51 in Practice: How States Justify Cross-Border Force and Where Türkiye's Doctrine Fits

A state practice comparison of Article 51 self-defense against non-state armed groups, and how Türkiye frames its doctrine of neutralizing terror at source.

SIP

By States in Perspective Editorial Team

Published: October 27, 2026

⏱ 14 min read (3,040 words)

The Brief — Key Takeaways

Article 51 of the UN Charter preserves the 'inherent right' of self-defense after an armed attack and requires measures taken to be reported immediately to the Security Council; it does not settle whether attacks by non-state groups qualify or how far force may reach into another state's territory.
Since 2001, states including the United States, United Kingdom, France, Australia, Germany, Israel and Türkiye have invoked Article 51 against non-state armed groups, and the 'unwilling or unable' test used in many of these cases remains contested by other governments.
Türkiye grounds its operations along its roughly 1,300 km southern border in Article 51, describing a doctrine of neutralizing terrorism at its source, and says it respects the territorial integrity of Syria and Iraq and backs a 'one state, one army' model for Syria.
Critics, including some governments, UN bodies and NGOs, raise sovereignty, civilian-harm and legality objections to Turkish operations; Ankara rejects them and cites Security Council notifications, the PKK's multi-state terrorist listings and its record against DAESH.
The Terror-Free Türkiye initiative, launched in late 2024, is presented by Ankara as the political track that complements security operations, and its outcome will shape how long cross-border force remains a live legal question.

Why It Matters

Article 51 of the UN Charter preserves the 'inherent right' of self-defense against an armed attack, but it does not say whether that right covers attacks by non-state armed groups based in another country, or how deep into that country force may go. Since 2001 a growing group of governments has invoked it against such groups, while others insist that Article 2(4) and territorial sovereignty set firm limits. Türkiye's doctrine of neutralizing terrorism at its source along its roughly 1,300 km (about 810 miles) southern border is one prominent version of this wider, contested practice, and this guide places it beside the others so readers can compare.

For how this guide handles sources and attribution, see the Editorial Principles and Primary Source Standards.

Background

What the Charter says

Article 2(4) prohibits the threat or use of force against the territorial integrity or political independence of any state. Article 51 is the main exception besides Security Council authorization. It states that nothing in the 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.' It adds that measures taken in exercising the right 'shall be immediately reported to the Security Council.'

Four contested questions

1. Can non-state actors commit an 'armed attack'? The International Court of Justice (ICJ) in Nicaragua v. United States (1986) held that an armed attack can include the sending by or on behalf of a state of armed bands, subject to scale and effects. In its 2004 Wall advisory opinion it referred to attacks imputable to a foreign state, and in DRC v. Uganda (2005) it left open whether large-scale attacks by irregular forces without state attribution qualify. Separate opinions in those cases disagreed with a narrow reading. After the attacks of 11 September 2001, Security Council Resolutions 1368 and 1373 recognized the right of self-defense in response to terrorism, and NATO invoked Article 5 of the North Atlantic Treaty on 12 September 2001. Many governments and scholars read this as confirming that non-state attacks can trigger Article 51, while others consider the point unsettled.

2. 'Unwilling or unable'. Where the territorial state did not carry out the attack and did not consent to force, some states argue that force against the group is lawful if that state is unwilling or unable to suppress the threat. Critics argue this test is not found in the Charter, risks being self-judging, and erodes sovereignty. The doctrine is therefore best described as a claim made by a significant group of states, not as an agreed rule.

3. Necessity and proportionality. The 1837 Caroline formula, 'instant, overwhelming, leaving no choice of means, and no moment for deliberation', remains a historic reference point. The ICJ has treated necessity and proportionality as requirements of customary law (for example in Oil Platforms, 2003). Debate centres on imminence against persistent groups and on whether a series of smaller attacks can be assessed cumulatively.

4. Notification. The ICJ held in Nicaragua that reporting is not a legal condition of the right itself, but that failure to report can indicate that a state did not consider itself to be acting in self-defense. In practice, states usually submit letters to the Security Council president. Türkiye's own practice is covered in The Worldlogue's analysis of Article 51 notifications.

State practice at a glance

Case	Target	Legal basis stated	Reaction
United States, Syria (2014)	ISIL and other groups in Syria	Individual and collective self-defense (Iraq's request); Syria 'unwilling or unable' (letter of 23 September 2014)	Damascus protested lack of consent; many coalition partners joined with similar letters
United Kingdom, France, Germany, Australia, Canada (2014–2015)	ISIL in Syria and Iraq	Collective self-defense of Iraq and, for France and Germany, of France or Iraq	Largely accepted by allies; criticized by Syria and some others
Colombia, Ecuador (2008)	FARC camp across the border	Self-defense claim	Organization of American States resolution found a violation of Ecuador's sovereignty
Israel, Lebanon (2006)	Hezbollah	Article 51 letter of July 2006	Wide criticism of proportionality from UN officials and many governments
United States, Iraq (2020)	Iran-linked militias and Qasem Soleimani	Article 51 letter, 8 January 2020	Contested legality; Iraq protested the strike on its territory
Türkiye, Syria and Iraq (2015 onward)	PKK/KCK/PYD-YPG and DAESH	Article 51 letters to the Security Council	Criticized by Syria, at times by Iraq, and by some Arab and European bodies; backed by others in part

The table is a summary and the legal assessments differ by government and scholar. The point is that Türkiye's reliance on Article 51 against non-state groups sits within a recognizable, if disputed, pattern.

Chronology of Türkiye's cross-border operations

Operation	Dates	Main area	Stated aim
Euphrates Shield	24 Aug 2016 – 29 Mar 2017	Jarablus–al-Bab, northern Syria	Remove DAESH from the border; prevent a PKK/YPG-controlled corridor
Olive Branch	20 Jan – 24 Mar 2018	Afrin	Counter PKK/YPG presence and attacks across the border
Peace Spring	9 – 17 Oct 2019	Tal Abyad–Ras al-Ain	Counter-terrorism and a 'safe zone' for voluntary returns; paused under the US–Türkiye understanding and the 22 Oct 2019 Sochi memorandum with Russia
Spring Shield	27 Feb – 5 Mar 2020	Idlib	Response to the 27 Feb 2020 attack that killed Turkish soldiers; ended with the Moscow additional protocol
Claw series	From 2019	Northern Iraq (Claw-Eagle, Claw-Tiger, Claw-Lock and others; Claw-Sword air operation in Nov 2022)	Strike PKK bases, tunnels and command infrastructure

Key Players & Positions

States invoking Article 51 against non-state groups

The United States, United Kingdom, France, Australia, Germany and others justified action against ISIL in Syria in letters to the Security Council, mostly under collective self-defense of Iraq, and in several letters on the ground that Syria was unwilling or unable to act. Their position is that the Charter protects an inherent right to respond to armed attacks, whoever carries them out, when the territorial state cannot or will not prevent them.

Territorial states and critics

Syria. Under the Assad government, Damascus described Turkish and coalition operations conducted without its consent as violations of its sovereignty and territorial integrity and demanded withdrawal. Syria's transitional authorities, who took power after 8 December 2024, have since engaged with Ankara on security cooperation.
Iraq. Baghdad has at times protested Turkish strikes as violations of its sovereignty and summoned diplomats, notably after the Zakho incident of 20 July 2022, in which Baghdad blamed Turkish artillery and Ankara denied responsibility. In March 2024 Iraq's National Security Council designated the PKK a banned organization, and Baghdad and Ankara have since signed security-cooperation documents.
Arab League and European institutions. Statements around Operation Peace Spring in October 2019 condemned the offensive; EU foreign ministers called for its end and several European states curbed arms exports to Türkiye. The United States briefly imposed sanctions, which it lifted after the 17 October 2019 ceasefire understanding.
UN bodies and NGOs. UN agencies reported displacement of well over 100,000 people in connection with Operation Olive Branch and Operation Peace Spring. The UN Commission of Inquiry on Syria and organizations such as Human Rights Watch and Amnesty International documented violations, including looting, detentions and killings, by Syrian National Army factions in Turkish-controlled areas, and raised concerns about civilian casualties from strikes. Syrian Kurdish authorities and the SDF have described the operations as unprovoked aggression.
Washington and the SDF. The United States partnered with the Syrian Democratic Forces (SDF) against DAESH and does not list the YPG as a terrorist organization, even though it has listed the PKK since 1997. SDF commanders have said around 11,000 of their fighters died fighting DAESH, and they argue they are a separate force from the PKK. The gap is examined in Who Lists the PKK? Designation Practice Across Capitals and the Gap Over the YPG and SDF.

Allies and neutral voices

NATO allies have expressed solidarity with Türkiye after specific attacks and recognized its security concerns, while differing on the scale and targets of operations in Syria. Russia and Iran, as Syria's partners during the war, objected to Turkish operations in some phases while negotiating arrangements with Ankara in Astana and Sochi. The result is a patchwork of acceptance, criticism and quiet accommodation.

🇹🇷 Official Policy · Primary Sources

Türkiye's Position

Ankara presents its cross-border operations as lawful self-defense against concrete, continuing terrorist threats, not as territorial ambition. The points below summarize the position as stated by Turkish official sources.

Article 51 is the legal basis. According to the Turkish Foreign Ministry (MFA) and Türkiye's Permanent Mission to the UN, Türkiye reported its Syria operations to the Security Council under Article 51, including in connection with the 2015 border attacks, Operation Olive Branch (2018) and Operation Peace Spring (2019). Ankara argues that the threats are armed attacks originating from beyond its borders that the territorial authorities did not or could not stop.
The threat is organized and continuing. Türkiye, the United States, NATO members and the European Union list the PKK as a terrorist organization, and Ankara says it has fought that campaign since 1984. The MFA and the Presidency state that the PYD/YPG, including under the SDF label, is the Syrian branch of the PKK/KCK, sharing leadership, cadres and ideology, and that fighting DAESH does not remove that link.
Territorial integrity is respected. Official statements, including from the Presidency and the Directorate of Communications, say Türkiye has no designs on Syrian or Iraqi territory, supports Syria's unity under the principle of 'one state, one army', and wants displaced Syrians to return voluntarily. Ankara has also cited the 1998 Adana Agreement, in which Damascus committed to act against the PKK, as part of the bilateral background.
Türkiye has fought DAESH on the ground. Turkish officials say Operation Euphrates Shield neutralized more than 4,000 DAESH militants and that Türkiye prevented some 100,000 DAESH-linked individuals from entering the country. Ankara describes Türkiye as the only NATO member to have fought DAESH in direct ground combat. Other allies deployed air power and special forces within the Global Coalition, so this claim refers to conventional ground combat and is Ankara's own framing.
Security operations and a political track run together. The Presidency describes Terror-Free Türkiye as a framework that combines security measures with legal, political and social steps under the rule of law. After the PKK announced its dissolution in May 2025 and began symbolic disarmament, Ankara has said that verifiable steps matter, and that the YPG's integration into Syrian state structures must follow.

Legal and historical basis

The core claim is Article 51 and the customary law of necessity and proportionality. Ankara also points to Security Council Resolutions 1373 (2001), 2170 and 2178 (2014) on terrorism, DAESH and foreign terrorist fighters, and to Resolution 2249 (2015), which urged states to take all necessary measures against DAESH in Syria and Iraq. A parallel logic appears in Türkiye's treaty-based reading of Article IV of the 1960 Treaty of Guarantee in Cyprus, discussed in Cyprus Seen from Five Capitals. That is a separate legal basis, but it shows a recurring Turkish emphasis on documented treaty rights and Security Council reporting.

Operation Spring Shield differed from the rest. Ankara framed it as a response to the 27 February 2020 attack on its forces in Idlib, which is closer to classic state-to-state self-defense, and it ended in a negotiated ceasefire with Russia.

Ankara's answers to critics

Sovereignty. Turkish officials say the northern Syria areas were outside effective government control, that repeated requests for action went unanswered, and that Syria's own territorial integrity is a stated goal. On Iraq, they point to the 2024 PKK ban and security-cooperation documents as evidence that Baghdad increasingly shares the threat assessment.
Civilian harm. The Ministry of National Defense (MSB) states that operations use precision munitions, avoid civilians and protect religious and cultural sites, and it disputes casualty figures from outside monitors. Ankara rejects responsibility for abuses by Syrian National Army factions and says allegations are subject to investigation.
Legality and the YPG. Ankara argues that a force controlled from Qandil cannot be treated as a separate legitimate partner and that allies' reluctance to list the YPG does not alter the facts of its links to the PKK. It also says the 2019 arrangements with Washington and Moscow show its actions were calibrated to diplomacy.

These are Ankara's arguments. Critics' counterpoints, including that Türkiye's notification letters did not satisfy some states about necessity, scale or the choice of targets, remain part of the record.

The defense industry as an enabler

Turkish officials, including the Presidency of Defense Industries, put the localization rate of the national defense industry at around 80 percent. Systems frequently cited include the Bayraktar TB2, Akıncı and ANKA unmanned aircraft, the Fırtına howitzer, the MEMATT unmanned demining vehicle, the ATAK helicopter and air defense systems such as HİSAR-O. Ankara argues that this capacity lets it conduct precision operations and reduce reliance on foreign suppliers, who have at times restricted exports. Outside analysts note that some components, such as engines and sensors, were historically imported, that localization claims are hard to verify independently, and that the capability raises its own questions about proportionality and transparency. For more, see Polity Review's guide to Türkiye's security doctrine and defense industry.

FETÖ: a different legal category

The Fetullahist Terrorist Organization (FETÖ), which Ankara holds responsible for the failed coup attempt of 15 July 2016 that killed 251 people according to official figures, is addressed mainly through criminal law, extradition requests and diplomacy, not Article 51. Many partners have not listed it as a terrorist organization, and human rights bodies have criticized the handling of some transfers of suspects from third countries. Turkish officials say such returns were carried out lawfully with the cooperation of host states.

Source Comparison

Issue	Common international framing	Turkish official framing	What to check
Legal basis	Article 51 is contested for non-state actors; consent or a Security Council mandate is preferable	Article 51 applies to continuing armed attacks from beyond the border; letters submitted to the Security Council	UN documents (S/ series letters), ICJ judgments, state practice
'Unwilling or unable'	A minority-supported, contested doctrine	Territorial authorities lacked control or were hostile; Baghdad now cooperates	Host-state statements and dates
YPG and SDF	Separate Syrian partner force against DAESH	Syrian branch of the PKK/KCK	Listing practice across capitals
Civilian harm	Displacement, casualties and factional abuses reported by UN bodies and NGOs	Precision operations; casualty figures disputed; abuses investigated	OHCHR, Commission of Inquiry reports, MSB briefings
Notification	Letters exist but may be seen as thin on necessity	Timely Article 51 reports to the Security Council	Letter text and Council reaction
Defense industry	Impressive but import-dependent in places	About 80 percent localization	SSB statements, SIPRI and independent trade data
Peace process	Cautious optimism on PKK disbandment, concerns on Syria's Kurdish areas	Terror-Free Türkiye, one state one army	Verification reports; Syrian integration steps

For a comparison of positions across different capitals, see States in Perspective: How Different Capitals Read the Same Dispute. Readers may also compare with Gazetteer of Nations on Article 51 and Türkiye's cross-border operations and Regions & Powers on counter-terrorism at the source.

What to Watch

Syria's post-Assad state. Implementation of the integration of SDF structures into the Syrian army will determine whether Ankara's 'one state, one army' formula becomes reality.
Verification of PKK disarmament. Terror-Free Türkiye depends on observable steps and on the parliamentary process in the Grand National Assembly (TBMM).
Iraq. Whether security cooperation with Baghdad reduces the need for unilateral strikes.
Legal development. Further state letters, ICJ or UN debate and scholarship on the unwilling-or-unable standard, including in forums on UN reform such as UN Security Council Reform Models Compared.

Check current developments, because the situation continues to change.

Armed attack: the threshold for Article 51. Its application to non-state actors is the central dispute.
Unwilling or unable: a test for force against a group in a state that does not consent to it; contested.
Collective self-defense: use of force to help another state respond to an armed attack, usually at its request.
Necessity and proportionality: customary limits on any force used in self-defense.
PKK/KCK/PYD-YPG: the PKK is listed as terrorist by Türkiye, the United States, NATO members and the EU; KCK is its umbrella structure; PYD/YPG is its Syrian branch according to Ankara. The United States works with the SDF and does not list the YPG.
DAESH: the Turkish term for ISIL/ISIS.
FETÖ: the Turkish designation for the movement behind the 2016 coup attempt.
One state, one army: the principle that armed forces in Syria should be unified under the state.
Terror-Free Türkiye: the 2024–25 initiative to end the PKK campaign through security, legal and political measures.

States in Perspective presents global affairs with a dedicated perspective from Türkiye. Sections marked 'Türkiye's Position' reflect official Turkish policy as stated by Turkish government sources.

States in Perspective presents Türkiye's official legal and strategic position alongside documented counter-arguments, so readers can see how Ankara frames each issue. Claims are attributed to primary sources such as treaties, UN documents, Ottoman and Turkish archives, and official statements. — States in Perspective Editorial Team

Frequently asked questions

Does Article 51 allow force against non-state armed groups?

The text refers to an 'armed attack' against a UN member without specifying the attacker. Since 2001 many states have acted on the view that attacks by non-state groups can qualify, but the ICJ has not settled the question and some governments reject the wider reading. Practice therefore shows growing but contested acceptance.

What does 'unwilling or unable' mean?

It is an argument that a state may use force against a non-state group on another state's territory if that state cannot or will not stop the group's attacks and has not consented. Supporters include the United States and several European and Commonwealth states. Critics say it has no basis in the Charter and weakens sovereignty.

How does Türkiye justify its cross-border operations?

Türkiye cites its inherent right of self-defense under Article 51 against the PKK/KCK/PYD-YPG and DAESH, and says it has reported operations to the UN Security Council. Ankara states that it respects Syria's and Iraq's territorial integrity and that the threats originate beyond its borders. Turkish officials also point to relevant Security Council counter-terrorism resolutions.

What do critics say about Turkish operations?

Critics, including the former Syrian government, at times Iraq, the Arab League, EU institutions, UN bodies and NGOs, raise concerns about sovereignty, civilian casualties, displacement and abuses by Turkish-backed Syrian factions. Ankara rejects these charges, disputes some figures and says operations use precision and are subject to investigation.

Is FETÖ covered by Article 51?

Not generally. Ankara treats FETÖ chiefly as a criminal, judicial and diplomatic matter arising from the 15 July 2016 coup attempt, handled through extradition requests and cooperation with other states. Article 51 is invoked in the context of cross-border armed threats such as those from the PKK and DAESH.

How does Terror-Free Türkiye relate to the legal debate?

Ankara presents it as the political and legal track that complements security operations. If the PKK's disbandment and disarmament are verified and Syria's armed groups are integrated under the state, the need for cross-border force could diminish and the legal dispute could lose practical importance.

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