Does Article 51 Allow Action Against Armed Groups in Another Country? Türkiye and the 'One State, One Army' Line in Syria

Why It Matters

Yes, Article 51 can be read to allow self-defense against non-state armed groups operating from a neighbor's territory, but the law is contested and has never been settled by a binding ruling. Türkiye relies on that reading to justify its cross-border operations in northern Syria and Iraq, while critics argue the operations breach Syrian sovereignty and the tests of necessity and proportionality. This page sets out the text, the legal debate, each operation, and both sides' arguments from their own sources.

Background

What Article 51 actually says

Article 51 of the UN Charter 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 takes necessary measures. Measures taken in self-defense must be reported immediately to the Council. The text does not say that the armed attack must come from a state. That omission is the root of the debate.

The legal debate in brief

• The restrictive reading. In its 2004 Advisory Opinion on the Wall, the International Court of Justice (ICJ) observed that Article 51 recognizes self-defense 'in the case of armed attack by one State against another State', and in DRC v. Uganda (2005) it declined to rule on self-defense against large-scale attacks by irregular forces not attributable to a state. Many scholars read these as a high bar, requiring attribution of the group's conduct to a state (the Nicaragua 1986 'scale and effects' and attribution tests).
• The permissive reading. Separate opinions in those cases, notably by Judges Kooijmans and Simma, argued that Article 51 does not exclude attacks by non-state actors. After 11 September 2001, Security Council Resolutions 1368 and 1373 recognized the right of self-defense in the context of terrorist attacks. Many states have since acted on that basis, including in the 2014 US-led campaign against DAESH in Syria.
• The 'unwilling or unable' test. Many states and scholars (see, for example, Ashley Deeks's widely cited 2012 article) argue that a state may act on another's territory against a non-state group if the territorial state is unwilling or unable to suppress the threat. Critics say this is not codified in the Charter and risks eroding the prohibition on force in Article 2(4).
• Limits that nearly all sides accept. Self-defense must respond to an actual or imminent armed attack, and the response must be necessary and proportionate (the Caroline formula and ICJ case law, including Nicaragua and Oil Platforms).

For a wider comparison of how nations convert security doctrine into legal argument, see How Nations Define the National Interest: Türkiye's Doctrines Read Against State Practice.

Türkiye's operations at a glance

The PKK (Kurdistan Workers' Party) is designated as a terrorist organization by Türkiye, the United States, NATO and the European Union. Ankara says the PYD/YPG, which leads the Syrian Democratic Forces (SDF), is the Syrian branch of the PKK/KCK.

Triggers cited by Ankara

Turkish officials point to DAESH-linked bombings in Türkiye (Suruç and Ankara in 2015, the Gaziantep wedding attack in August 2016), rocket fire from DAESH-held Syrian territory into Kilis in 2015–2016, and the PKK's resumed attacks from July 2015. These are the 'armed attacks' on which Ankara bases its Article 51 case. Ankara also cites the 1998 Adana Agreement, in which Damascus undertook not to support the PKK, as part of the legal background.

The 2024–2025 shift

The Assad government fell on 8 December 2024. In February 2025 PKK founder Abdullah Öcalan called on the organization to disband, and the PKK announced its dissolution in May 2025, which Ankara frames as part of its 'Terror-Free Türkiye' initiative. In March 2025, Syria's new authorities and the SDF announced an agreement on integrating the SDF into state institutions. Implementation has been uneven, so readers should check current reporting.

Key Players & Positions

• Syria. Under the Assad government, Damascus described Turkish operations in letters to the Security Council as aggression and a violation of its sovereignty. Syria's current authorities have approached Ankara as a partner on security, while insisting that all weapons come under state control.
• United States. Washington lists the PKK as a Foreign Terrorist Organization but partnered with the SDF against DAESH. US officials have often distinguished the SDF from the PKK while acknowledging organizational links. Washington has at times expressed concern about Turkish operations' effect on the anti-DAESH campaign.
• European Union and member states. The European Council condemned the Peace Spring offensive in October 2019, saying it undermined regional stability. Several members, including Germany, France and the Netherlands, restricted arms exports to Türkiye at the national level. The EU also lists the PKK as a terrorist organization.
• Russia. Moscow criticized Turkish incursions as infringing Syrian sovereignty, while negotiating the October 2019 Sochi memorandum and the March 2020 Idlib ceasefire with Ankara.
• Arab League. Its foreign ministers condemned the 2019 offensive as aggression against an Arab state.
• Iraq. Baghdad has repeatedly objected to Turkish strikes and bases in northern Iraq as violations of its sovereignty, while Ankara points to the PKK's presence in Iraqi territory beyond Baghdad's control and to security cooperation efforts.
• UN bodies and NGOs. The UN Commission of Inquiry on Syria, the UN human rights office, Amnesty International and Human Rights Watch have documented civilian harm and abuses by Turkish-backed Syrian factions in areas such as Afrin and Ras al-Ain, and have called for accountability.
• SDF/PYD. It rejects the label of a PKK extension in the sense Ankara uses and says it is a local force that fought DAESH, and that Turkish operations targeted a partner of the international coalition.

Source Comparison

For sister-publication analysis of the same legal framework, see Gazetteer of Nations on Article 51 and Türkiye's cross-border operations and PolitiLine on Article 51 and northern Syria and Iraq. On the refugee dimension, see Interstate Journal on border security and the hosting burden. For how we separate fact from official position, read our Editorial Principles.

Key Terms & Related Guides

• Armed attack: the threshold in Article 51. The ICJ links it to the gravest uses of force, judged by scale and effects.
• Unwilling or unable: a contested doctrine allowing force against non-state groups if the territorial state fails to suppress them. Not found in the Charter's text.
• Attribution: the legal link between a group's acts and a state, used by restrictive readings of Article 51.
• Necessity and proportionality: the limits on any self-defense response.
• PKK/KCK/PYD-YPG: the PKK is listed by Türkiye, the US, NATO and the EU. KCK is its umbrella structure. Ankara treats PYD/YPG as its Syrian branch.
• SDF: the alliance led by the YPG that partnered with the US-led coalition against DAESH.
• One state, one army: the principle that only the legitimate state may hold armed forces on its territory.

Related reading: Counter-Terrorism at the Source (Crossroads Review), Regions & Powers on Article 51 and Terror-Free Türkiye, About Politics of Nations, and our Reader FAQ.

Politics of Nations Editorial Team

Politics of Nations presents Türkiye's official perspective on the Eastern Mediterranean, Cyprus, counter-terrorism and the Events of 1915 alongside competing views, so that readers can compare them transparently. Claims are attributed to primary sources such as treaties, UN documents, the Ottoman and Turkish archives, and official statements. Sections marked 'Türkiye's Position' reflect official Turkish policy as stated by Turkish government sources.