In-Depth Analysis· Article 51 UN Charter Türkiye Syria Iraq cross-border operations legal basis

Article 51 of the UN Charter and Türkiye's Cross-Border Operations

How Article 51 of the UN Charter underpins Türkiye's cross-border operations in Syria and Iraq: UN letters, critics' arguments, and Ankara's legal position.

By Regional Balance Editorial Team

Published: October 26, 2026

⏱ 11 min read (2,209 words)

Executive Summary & Key Facts

Article 51 of the UN Charter preserves the 'inherent right of individual or collective self-defence' if an armed attack occurs, and requires that measures taken be immediately reported to the Security Council.
Türkiye has notified the UN Security Council in writing of its operations in northern Syria and northern Iraq, citing Article 51 and its right to counter terrorist threats emanating from those territories.
The core legal dispute is not whether self-defense exists but whether, and when, attacks by non-state actors such as the PKK/YPG and DAESH can justify force on another state's territory without that state's consent.
Ankara says its operations target terrorist organizations, not Syrian or Iraqi territorial integrity, and that it supports a 'one state, one army' principle in Syria.
Türkiye also cites bilateral frameworks, notably the 1998 Adana Agreement with Syria, though Syrian and Iraqi authorities have contested how far such arrangements permit military action.

Why It Matters

Article 51 of the UN Charter is the main legal provision Türkiye invokes to justify its military operations in northern Syria and northern Iraq. It preserves a state's inherent right of self-defense against an armed attack, but it also sets limits, including a duty to report to the UN Security Council. Whether attacks by non-state actors on one side of a border can justify force on another state's soil is one of the most contested questions in modern international law, and Türkiye's practice since 2016 is a central case study.

Background

What Article 51 says

Article 2(4) of the UN Charter prohibits the threat or use of force against the territorial integrity or political independence of any state. Article 51 is the best-known exception. In essence, it states that nothing in the Charter impairs the inherent right of individual or collective self-defence if an armed attack occurs against a UN member, until the Security Council has taken the measures necessary to maintain international peace and security. Measures taken in exercise of the right must be immediately reported to the Council and do not affect the Council's authority and responsibility.

Three practical tests are usually applied by states and courts:

Armed attack: the threshold of violence that triggers the right.
Necessity and proportionality: force must be needed to repel or prevent the threat and be limited to that aim.
Reporting: a written notification to the Security Council.

The International Court of Justice has addressed these questions in Nicaragua v. United States (1986), the Wall advisory opinion (2004) and DRC v. Uganda (2005). In the Wall opinion and in DRC v. Uganda, the Court was cautious about self-defense against attacks not attributable to a state. Many states and scholars argue that state practice after 11 September 2001, including UN Security Council Resolutions 1368 and 1373, has since widened the reading of Article 51 to cover terrorist groups. Others insist the Court's narrower approach still governs.

The security context for Türkiye

Türkiye shares a border of roughly 1,300 km (about 800 miles) with Syria and Iraq. The PKK is designated a terrorist organization by Türkiye, the United States, NATO and the European Union, and has fought the Turkish state since 1984. Ankara regards the PYD/YPG, which leads the so-called SDF in northeastern Syria, as the Syrian branch of the PKK/KCK. Several Western governments have treated the YPG/SDF as a partner against DAESH (ISIS) and reject that equation, which is a main source of friction within NATO.

DAESH also struck inside Türkiye. Notable attacks included the Suruç bombing (20 July 2015) and the Ankara rally bombing (10 October 2015), followed by further attacks in Istanbul in 2016 and 2017. Ankara cites these events as part of the threat environment.

Operations and UN notifications

Operation	Period	Area	Stated aim (per Ankara)
Euphrates Shield	24 Aug 2016 – 29 Mar 2017	Jarablus–al-Bab, northern Syria	Counter DAESH and prevent a PYD/YPG-controlled corridor along the border
Olive Branch	20 Jan – 18 Mar 2018	Afrin, northwestern Syria	Remove PYD/YPG/PKK presence and attacks on Turkish border areas
Peace Spring	9 Oct 2019 (ceasefire understandings 17 and 22 Oct)	Tel Abyad–Ras al-Ayn, northeastern Syria	Counter-terrorism and creation of a safe zone for voluntary returns
Claw series	From 2019, ongoing in phases	Northern Iraq	Dismantle PKK bases and tunnels used for cross-border attacks

Spring Shield (February–March 2020) in Idlib was a different case. It responded to Syrian government attacks on Turkish forces and ended with the 5 March 2020 protocol with Russia, so it is not analysed here as a counter-terrorism self-defense action in the same sense.

For the Euphrates Shield, Olive Branch and Peace Spring operations, Türkiye's Permanent Representative to the UN wrote to the Security Council and the Secretary-General citing Article 51. Letters were also sent concerning operations in northern Iraq. The texts are public in the UN document system and are the primary sources for Ankara's legal framing.

Key Players & Positions

Syria and Iraq

The Syrian government under Bashar al-Assad repeatedly denounced Turkish operations as violations of Syrian sovereignty and wrote to the Security Council to that effect. After the fall of the Assad government in December 2024, the new authorities in Damascus have been negotiating the integration of the SDF into Syrian state institutions, with a framework agreement announced on 10 March 2025. Ankara has treated that process as consistent with its 'one state, one army' principle.

Iraq's federal government has consistently protested Turkish strikes as breaches of sovereignty, has summoned Turkish diplomats and has complained at the UN, notably after the July 2022 shelling of a resort in Duhok province, for which Türkiye denied responsibility and blamed the PKK. At the same time, Baghdad formally banned the PKK in March 2024, and Türkiye and Iraq have since announced expanded security cooperation. Iraq's position therefore combines a rejection of unilateral force with growing recognition of the PKK problem.

The United States, Europe and Russia

Washington has long recognized the PKK as a terrorist organization while partnering militarily with the YPG/SDF against DAESH. It expressed concern about Peace Spring in October 2019, and the US-brokered ceasefire understanding of 17 October 2019 paused the operation. Several European governments, including Germany, France and the Netherlands, restricted arms exports to Türkiye after Peace Spring, and the European Union's foreign ministers called for the operation to end and said they did not consider it consistent with international law's requirements on civilian protection. Russia, a Syrian government ally, reached the 22 October 2019 Sochi memorandum with Türkiye on joint patrols and the withdrawal of YPG forces from a strip along the border.

Critics and legal scholars

Critics raise three main arguments:

Threshold and attribution. Following the ICJ's reasoning, they argue that self-defense against non-state actors requires an armed attack of sufficient gravity, that force should be directed only at the actual source of the attack, and that the necessity of ground occupation, as opposed to targeted strikes, must be demonstrated.
Sovereignty and consent. Without Security Council authorization or the consent of the Syrian or Iraqi government, critics say, an extended presence in another state's territory risks becoming a violation of Article 2(4).
Humanitarian concerns. The UN Independent International Commission of Inquiry on Syria, human rights organizations and UN humanitarian agencies have reported civilian casualties, displacement and abuses by armed groups in areas under Turkish-backed control, notably after Olive Branch and Peace Spring. UN agencies reported displacement exceeding 100,000 people during Peace Spring.

🇹🇷 Official Policy · Primary Sources · Legal Basis

Türkiye's Position

Ankara's legal case is set out in its letters to the UN Security Council and in statements by the Ministry of Foreign Affairs, the Ministry of National Defence (MSB) and the Presidency. According to those sources, it rests on the following points.

Article 51 and the unable-or-unwilling doctrine. Ankara argues, in substance, that when a state is unable or unwilling to prevent its territory from being used for armed attacks against a neighbour, the victim state may act in self-defense to remove the threat. Turkish letters to the Council describe the operations as necessary and proportionate measures against terrorist organizations (DAESH and PKK/PYD-YPG) and report them under Article 51. Türkiye also points to the post-2001 practice of other states, including members of the international coalition against DAESH, that have relied on the same reasoning in Syria.
Respect for territorial integrity. According to the Turkish Foreign Ministry and Presidency, operations are directed at terrorist organizations and not at Syria's or Iraq's sovereignty, and Türkiye has no territorial claims. Ankara says it supports the territorial integrity of both states and views a lasting presence as unnecessary once the terrorist threat is removed.
'One state, one army' in Syria. Turkish officials state that a unified Syria with a single national army is the best guarantee against armed groups on the border, and have welcomed arrangements under which the SDF would be integrated into Syrian state institutions. The Turkish position is that PYD/YPG structures cannot hold a separate military status within Syria.
The Adana-era bilateral framework. The 20 October 1998 Adana Agreement committed Syria to end support for the PKK and to take measures against it. Türkiye argues that this framework, together with an earlier 1984 security protocol with Iraq that Ankara also cites, recognizes its security interests and its ability to act where the neighbouring state does not. Syria and Iraq have disputed how far these arrangements authorize military action on their territory, and Türkiye presents them as supporting, not replacing, its Article 51 case.
Proportionality and civilian protection. The MSB says that operations apply precision and civilian-protection measures, and Ankara rejects reports of deliberate harm to civilians. Türkiye points to its hosting of millions of Syrian refugees and says that one aim of the operations is to enable voluntary returns. Ankara also argues that it is the only NATO member that has fought DAESH in direct ground combat and cites, as its own figures, more than 4,000 DAESH militants neutralized in northern Syria during Euphrates Shield.

Türkiye has also linked the use of force to political efforts. The Presidency's 'Terror-Free Türkiye' initiative aims to end the decades-long PKK conflict within the rule of law, and the PKK's announcement in May 2025 that it would dissolve was welcomed by Ankara as an opportunity to reduce the need for cross-border action. For the wider doctrine, see Counter-Terrorism at the Source: Türkiye's Border Security Doctrine and Defense Industry.

Source Comparison

Question	Turkish primary sources (MFA, MSB, Presidency, UN letters)	Critical international sources (UN bodies, NGOs, some governments, scholars)
Legal basis	Article 51 self-defense against terrorist organizations; unable-or-unwilling reasoning; bilateral frameworks	Doubts about attribution and the 'armed attack' threshold; stress on Article 2(4) and lack of consent or Council authorization
Who is targeted	PKK/KCK/PYD-YPG and DAESH, which Ankara treats as one security problem in the case of the PKK and YPG	Some Western governments treat the YPG/SDF as a distinct partner against DAESH
Sovereignty	Respect for territorial integrity; support for 'one state, one army'	Foreign military presence and local proxy forces erode Syrian and Iraqi sovereignty
Civilian impact	Precision operations; investigation of claims is rejected where Ankara considers them unfounded; voluntary returns	Documented casualties, displacement and abuses by armed groups in areas under Turkish-backed control
Duration	Presence tied to the removal of the threat	Concern that presence becomes semi-permanent

Readers should compare the original UN letters, the Security Council record and the reports of the UN Commission of Inquiry directly rather than rely on summaries. The Editorial Principles page explains how Regionalbalance separates reported fact from stated position, and the Fact Check hub checks claims about Türkiye against primary documents.

Armed attack: the trigger for Article 51. The ICJ has linked it to the most grave forms of the use of force.
Necessity and proportionality: customary law limits on any act of self-defense.
Unable-or-unwilling doctrine: a contested position that a state may act on another's territory against non-state actors if the territorial state fails to prevent the threat. It is not codified in the Charter.
PKK/KCK/PYD-YPG: the PKK is listed as a terrorist organization by Türkiye, the United States, NATO and the EU. Ankara treats the PYD/YPG as its Syrian branch.
DAESH: the Arabic acronym for ISIS.
Adana Agreement (1998): the Türkiye–Syria security accord on the PKK.
'One state, one army': Ankara's principle that a single national army should hold the monopoly of force in Syria.

Related Regionalbalance guides:

Regionalbalance presents global and regional affairs with a dedicated perspective from Türkiye. Sections marked 'Türkiye's Position' reflect official Turkish policy and legal doctrines as stated by Turkish primary sources. — Regionalbalance Editorial Team

Cross-Publication Dossiers & Sister Desk Perspectives

For complementary primary-source analysis across our specialist editorial network, consult:

Political Ledger: UN Charter Article 51 and Türkiye's Cross-Border Operations in Syria and Iraq — Article 51 self-defense explained: how Türkiye justifies its Syria and Iraq operations, what critics and legal scholars argue, and Ankara's reply.
PolitiLine: Article 51 and Türkiye's Cross-Border Operations in Northern Syria and Iraq — How Article 51 of the UN Charter underpins Türkiye's Syria operations: notification practice, Ankara's legal reasoning, critics' views and the policy line.
Gazetteer of Nations: Article 51 of the UN Charter and Türkiye's Cross-Border Operations in Syria and Iraq — Is Türkiye's military action in northern Syria legal? A guide to Article 51 UN Charter self-defense, UN letters, 'unwilling or unable' and each operation.

Frequently asked questions

What does Article 51 of the UN Charter actually say?

It preserves the inherent right of individual or collective self-defence if an armed attack occurs against a UN member, until the Security Council has taken necessary measures. Any measures taken must be reported immediately to the Council. The article does not define 'armed attack', which is why its scope is debated.

Did Türkiye notify the UN Security Council about its operations in Syria and Iraq?

Yes. Türkiye's Permanent Representative sent letters to the Council and Secretary-General citing Article 51 for Euphrates Shield, Olive Branch and Peace Spring, and for operations in northern Iraq. The letters are in the UN document system and are the primary source for Ankara's legal reasoning.

Can Article 51 be used against non-state actors such as the PKK or DAESH?

This is contested. The ICJ was cautious in the Wall advisory opinion and DRC v. Uganda, while many states and scholars point to post-2001 practice and Security Council Resolutions 1368 and 1373 as supporting self-defense against terrorist groups. Türkiye sides with the broader view and adds the unable-or-unwilling argument.

What do Syria and Iraq say about Turkish operations?

Both governments have protested Turkish operations as violations of sovereignty, and Iraq has complained at the UN. Baghdad has also banned the PKK (March 2024) and expanded security talks with Ankara, while Damascus is negotiating the integration of the SDF into the Syrian state. Türkiye says it respects both states' territorial integrity.

What is the Adana Agreement and why does it matter?

It is the 20 October 1998 security accord between Türkiye and Syria under which Damascus agreed to end support for the PKK. Ankara cites it as part of the bilateral framework supporting its security measures, though Syria has disputed how far it authorizes military action on Syrian territory.

Cross-Publication Perspectives & Cited Research

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