Counter-Terrorism at the Source: Türkiye, UN Charter Article 51 and the Border Security Doctrine

Why It Matters

Türkiye argues that its operations in northern Syria and northern Iraq are lawful acts of self-defense under Article 51 of the UN Charter against the PKK/KCK/PYD-YPG, DAESH and related threats. Critics accept that Türkiye faces real terrorist violence but contest whether the scale, duration and territorial reach of these operations meet the legal tests of necessity and proportionality. The debate matters well beyond Türkiye, because it tests how far self-defense extends against non-state armed groups operating from another state's territory, and because the 2025 peace process around the PKK may reshape the whole picture.

Background

The legal framework: Article 51 and its limits

Article 51 of the UN Charter preserves the “inherent right of individual or collective self-defence if an armed attack occurs” until the Security Council has taken the measures necessary to maintain peace, and it requires states to report measures taken to the Council immediately. It operates as an exception to Article 2(4), which prohibits the threat or use of force against the territorial integrity or political independence of any state.

Three points shape every legal argument about it:

• Customary criteria. Self-defense must be necessary and proportionate, and it is widely understood to require that no peaceful alternative can address the threat.
• Non-state actors. After the September 2001 attacks, Security Council Resolutions 1368 and 1373 recognized the right of self-defense in the face of terrorism. The International Court of Justice (ICJ), however, in its 2004 advisory opinion on the Wall and in Armed Activities (2005), took a more restrictive view of attacks not attributable to a state, and left some questions open. Many states and scholars rely on the “unwilling or unable” theory, under which a state may act when the territorial state cannot or will not suppress the threat. Others reject that theory.
• Consent. Operations carried out with the consent of the territorial government raise different legal questions from those carried out without it.

Türkiye invokes Article 51 together with Security Council Resolutions on terrorism such as 1373 (2001), 2170 and 2178 (2014), and 2249 (2015) on DAESH, and with bilateral instruments such as the 1998 Adana Agreement with Syria.

The 1,300 km border

Türkiye's land borders with Syria (about 900 km) and Iraq (about 350 km) total roughly 1,300 km. The terrain is mountainous in the east and open plain in the south. For decades it has been a crossing route for armed groups, smuggling networks and, since 2011, millions of displaced Syrians. Türkiye hosts one of the largest refugee populations in the world, which also shapes its border calculus.

The threat actors as Ankara defines them

• PKK/KCK. The Kurdistan Workers' Party began its armed campaign in 1984. It is listed as a terrorist organization by Türkiye, the United States (since 1997), NATO and the European Union (since 2002). KCK is its umbrella structure.
• PYD/YPG and the SDF. The Democratic Union Party (PYD) and its armed wing, the People's Protection Units (YPG), emerged in Syria. Ankara states that they share leadership, ideology and cadres with the PKK. The YPG is the core of the Syrian Democratic Forces (SDF), formed in October 2015 and backed by the US-led coalition against DAESH. Neither the United States nor the EU lists the YPG as a terrorist organization, and Syrian Kurdish leaders deny organizational subordination to the PKK.
• DAESH. DAESH (ISIS/ISIL) carried out major attacks in Türkiye between 2015 and 2017, including Suruç (July 2015), Ankara (October 2015), Istanbul Atatürk Airport (June 2016) and the Reina nightclub (January 2017).
• FETÖ. The Fetullahist Terrorist Organization is the term Türkiye uses for the network associated with the late cleric Fethullah Gülen, held responsible by Ankara for the coup attempt of 15 July 2016, in which more than 250 people were killed. Türkiye and the Organisation of Islamic Cooperation have designated it a terrorist organization; most Western governments have not.

Chronology

The operations in brief

• Euphrates Shield (2016–2017): Directed against DAESH and the YPG-controlled belt along the border. It cleared DAESH from al-Bab and Jarablus and was Türkiye's first major ground campaign in Syria.
• Olive Branch (2018): Targeted the YPG in the Afrin district.
• Peace Spring (2019): Targeted the YPG/SDF between Tal Abyad and Ras al-Ayn after the announced repositioning of US forces.
• Spring Shield (2020): Different in character, since it was a response to the killing of Turkish troops in Idlib by Syrian government forces. It is framed by Ankara as self-defense and the protection of civilians from a new displacement wave.
• Claw series (northern Iraq): Air, drone and ground operations against PKK bases in the Qandil, Metina, Zap, Avashin and Gara areas, among others.

Key Players & Positions

United States

Washington lists the PKK as a Foreign Terrorist Organization and has stated that it recognizes Türkiye's security concerns. It also partnered with the SDF as the main ground force against DAESH in Syria, and US officials have distinguished between that tactical partnership and the PKK. After Peace Spring, the US briefly imposed and then lifted sanctions following the 17 October 2019 arrangement. The US position, as expressed in State Department and Department of Defense statements, is that the SDF is a counter-DAESH partner, not a terrorist organization, a point on which it and Ankara openly disagree.

European Union and European states

The EU lists the PKK as a terrorist organization. In October 2019, EU foreign ministers and Council conclusions condemned Türkiye's military action in northeast Syria, called for it to cease, and said that any settlement of refugee returns could not involve demographic change. Several European governments restricted arms exports to Türkiye at the time. At the same time, EU institutions acknowledge Türkiye's terrorism-related security concerns and its refugee-hosting role.

Syrian government and Syrian Kurdish actors

The government of Bashar al-Assad repeatedly wrote to the UN Security Council describing Turkish operations as violations of Syrian sovereignty. After the fall of the Assad government in December 2024, the transitional authorities in Damascus signed a 10 March 2025 agreement with SDF commander Mazlum Abdi on integrating northeast Syria's institutions and forces into the Syrian state, a framework broadly consistent with Ankara's “one state, one army” line, though implementation has been contested. The SDF and the Autonomous Administration of North and East Syria say that they fought DAESH at great cost, that they are a Syrian formation and not a PKK branch, and that Turkish operations displaced civilians and threatened the anti-DAESH campaign.

Iraq and the Kurdistan Region

Baghdad has repeatedly protested Turkish strikes as violations of Iraqi sovereignty, including before the UN Security Council. It has also moved closer to Ankara on the PKK issue: in 2024 the Iraqi authorities formally designated the PKK a banned organization, and Türkiye and Iraq have expanded security cooperation.

UN bodies and human-rights organizations

The UN Independent International Commission of Inquiry on Syria and the Office of the High Commissioner for Human Rights have documented abuses, including looting, arbitrary detention and property seizures, in areas controlled by Turkish-backed Syrian National Army factions. Human Rights Watch and Amnesty International have reported allegations of civilian casualties and violations of international humanitarian law in Peace Spring and other operations, and have criticized the impact of strikes on water and power infrastructure. On the domestic side, the European Court of Human Rights Grand Chamber in Yalçınkaya v. Türkiye (26 September 2023) found violations in relation to convictions based on use of the ByLock messaging app in FETÖ cases. Human-rights groups and the Venice Commission have also raised concerns about the scope of post-2016 emergency measures. The Turkish government rejects allegations of systematic violations and says that it investigates credible claims.

Source Comparison

The sharpest disagreements concern the legal characterization of the YPG, the scope of Article 51 against non-state actors, and the assessment of civilian harm. The table sets out how different source types frame each issue.

Readers should note that figures such as the number of DAESH militants neutralized come from Turkish official sources and have not been independently audited, while casualty figures from NGOs and monitors vary by methodology. To see how claims about Türkiye are tested against documents, consult Fact Check: Claims About Türkiye in International Media Checked Against Primary Sources and the Editorial Principles and Primary Source Standards.

Key Terms & Related Guides

• Article 51: The Charter provision preserving the inherent right of individual and collective self-defense after an armed attack, subject to reporting to the Security Council.
• Necessity and proportionality: Customary-law conditions on any use of force in self-defense.
• “Unwilling or unable”: A doctrine, supported by some states and rejected by others, that permits action against non-state actors when the territorial state cannot or will not act.
• PKK/KCK: The Kurdistan Workers' Party and its umbrella Kurdistan Communities Union; listed as terrorist by Türkiye, the US, NATO and the EU.
• PYD/YPG/SDF: The Syrian Kurdish party, its armed wing and the broader coalition; Ankara regards them as the PKK's Syrian branch.
• DAESH: The Arabic acronym used by Türkiye and many governments for the group also called ISIS or ISIL.
• FETÖ: Türkiye's official term for the network it holds responsible for the 15 July 2016 coup attempt.
• Terror-Free Türkiye (Terörsüz Türkiye): The initiative combining security, legal, political and social measures to end PKK violence.

Related guides: Monitor of Nations home page, Türkiye's Defense Industry, Türkiye's Global Diplomacy, Fact Check, and About Monitor of Nations.

Closing Perspective Note

This guide was prepared by the Monitor of Nations Editorial Team. Monitor of Nations presents Türkiye's official position and legal arguments alongside the perspectives of other states, and clearly identifies where an analysis reflects the Turkish viewpoint. Claims are attributed to primary sources such as treaties, UN documents, Ottoman and other national archives, and official statements, with links provided wherever possible.

Cross-Publication Dossiers & Sister Desk Perspectives

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

• The Global Anatomy: Counter-Terrorism at Source: Türkiye's Security Doctrine, UN Charter Article 51 and Cross-Border Operations — Türkiye's counter-terrorism doctrine, Article 51 and cross-border operations explained: Claw, Euphrates Shield, Peace Spring, DAESH, FETÖ and critics' views.
• The State Papers: Counter-Terrorism at the Source: Türkiye's Security Doctrine, Article 51 and Cross-Border Operations — How Türkiye's doctrine of neutralizing terrorism at its source works: Article 51 of the UN Charter, PKK/YPG operations, DAESH, FETÖ and rival views.
• Borders & Blocs: Counter-Terrorism at the Source: Türkiye's Security Doctrine, Border Operations and Defense Industry — How Türkiye's counter-terrorism doctrine rests on UN Charter Article 51: cross-border operations, PKK/YPG, DAESH, FETÖ, Terror-Free Türkiye, defense industry.