Applying the Article 51 Test: Necessity, Proportionality and Türkiye's Cross-Border Operations

Why It Matters

Article 51 of the UN Charter allows force in self-defense only when an armed attack occurs, and customary international law requires that the force be both necessary and proportionate. Türkiye argues that its operations in northern Syria and northern Iraq meet this test because PKK/KCK/PYD-YPG and DAESH attacked and threatened its territory from areas where no effective authority could stop them. Critics, including Syria, Iraq and several human rights bodies, reply that some operations moved from defense to territorial control and caused civilian harm. This guide sets out the test, applies it operation by operation, and presents both sides with attribution.

Background

What Article 51 and customary law require

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 main exception: '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 must be 'immediately reported' to the Security Council.

Four questions follow from this text and from case law:

• Armed attack: Was there an attack of sufficient gravity? The International Court of Justice (ICJ) in Nicaragua v. United States (1986) distinguished 'the most grave forms of the use of force' from lesser incidents.
• Necessity: Was force required, with no reasonable peaceful alternative? The classical formulation comes from the 1837 Caroline correspondence, which spoke of a necessity that is 'instant, overwhelming, leaving no choice of means, and no moment for deliberation'.
• Proportionality: Was the force limited to what is needed to repel or end the threat? The ICJ applied this in Oil Platforms (2003).
• Reporting: Was the Security Council notified promptly?

Non-state actors and the 'unable or unwilling' debate

The hardest question is whether self-defense extends to attacks by non-state actors operating from another state's territory. After 11 September 2001, Security Council resolutions 1368 and 1373 recognized the right of self-defense in the terrorism context and obliged states to deny safe haven to terrorists. Later resolutions, including 2170 and 2178 (2014) and 2249 (2015), addressed DAESH, foreign terrorist fighters and the need for 'all necessary measures' against DAESH in Syria and Iraq.

The ICJ has been more cautious. In its 2004 Wall advisory opinion it referred to Article 51 in terms of attacks by one state against another, and in DRC v. Uganda (2005) it left open whether self-defense is available against large-scale attacks by irregular forces not attributable to a state. State practice moved further. In 2014 the United States told the Security Council it was acting against DAESH in Syria because the Syrian government was 'unable or unwilling' to confront the threat. That standard, developed in the 2012 Bethlehem principles, is invoked by a growing number of states but remains contested. Its critics argue that it can dilute the consent requirement of territorial sovereignty. For a sister-publication view of how Article 51 has been applied to these operations, see Gazetteer of Nations on Article 51 and Türkiye's cross-border operations.

The security setting

Türkiye shares a land border of roughly 1,300 km (about 810 miles) with Syria and Iraq combined, around 900 km with Syria and 350 km with Iraq. The PKK, founded in 1978, has waged an armed campaign against Türkiye since 1984. Türkiye, the United States (1997), the European Union (2002) and NATO list it as a terrorist organization. DAESH (ISIS) conducted or inspired mass-casualty attacks in Türkiye in 2015 and 2016, including the Suruç bombing (2015-07-20), the Ankara rally bombing (2015-10-10) and the Gaziantep wedding bombing (2016-08-20). Shelling from Syrian territory hit Turkish border towns such as Kilis.

Türkiye's earlier legal references include the 1998 Adana security arrangement with Syria, in which Damascus undertook to stop PKK activity on its territory. Ankara has also cited a 1984 border-security protocol with Iraq, which Baghdad has contested.

Chronology of the operations

Türkiye described each operation as self-defense under Article 51 in its Security Council communications, notably letters in 2015, 2018 and 2019. Spring Shield differs structurally because it answered an attack attributed to Syrian government forces rather than a non-state actor.

Key Players & Positions

Syria

Until December 2024 the Syrian government repeatedly told the Security Council that Turkish operations violated its sovereignty and territorial integrity and constituted aggression, rejecting Ankara's self-defense claims. After the change of government in Damascus in December 2024, the transitional authorities have prioritized negotiating the integration of the SDF into state institutions (the 2025-03-10 agreement with SDF commander Mazloum Abdi is the best-known step). Türkiye states that this approach corresponds to its own 'one state, one army' principle.

Iraq

The federal government in Baghdad has long protested Turkish strikes as violations of Iraqi sovereignty and has raised them at the UN, including after the shelling of a resort at Perex near Zakho on 2022-07-20, which killed civilians. Ankara denied responsibility and blamed the PKK, proposing a joint investigation. Iraq's position has shifted. In 2024 its National Security Council classified the PKK as a banned organization, and the two governments announced expanded security cooperation. Baghdad nevertheless maintains that unilateral action on Iraqi soil requires its consent.

The SDF, the United States and the Global Coalition

The Syrian Democratic Forces (SDF) were the main ground partner of the US-led Global Coalition against DAESH, from Raqqa (2017) to the last DAESH enclave at Baghouz (March 2019). Washington has stated that the SDF's counter-DAESH role matters to its own security interests and has criticized Turkish operations that it believed risked the campaign against DAESH and the security of detention facilities. At the same time, the United States lists the PKK as a terrorist organization and has acknowledged the PKK's organizational links to the YPG. Türkiye is itself a Coalition member. The question of how to treat a partner force that Ankara sees as part of the PKK is the central transatlantic disagreement on this file.

Human rights bodies and international organizations

The UN Independent International Commission of Inquiry on Syria, OHCHR, Amnesty International and Human Rights Watch have reported civilian casualties, displacement, looting and abuses by armed groups affiliated with Turkish-backed forces in areas such as Afrin and Ras al-Ayn. These findings bear on proportionality and on the responsibilities of any force exercising control over territory. Turkish authorities reject allegations of deliberate harm to civilians. They say that they investigate reported incidents, that Turkish forces use precision munitions, and that civilians in Turkish border towns were also killed by fire from across the border.

Other states and organizations

The EU Council in October 2019 called on Türkiye to end the Peace Spring operation, and several members suspended arms exports. The Arab League condemned the operation as an infringement of Arab sovereignty. Russia, as the guarantor of the Damascus side in northeast Syria, brokered the 2019-10-22 Sochi memorandum with Türkiye on patrols and the withdrawal of YPG forces from the border strip.

What scholars argue

Legal commentary divides on three issues:

• Whether past attacks that have ended justify a later operation (the 'accumulation of events' theory and the time gap).
• Whether holding and administering territory for years is compatible with proportionality.
• Whether the 'unable or unwilling' standard has gained enough state support to be customary law.

Source Comparison

Where the test is strong and where it is contested

• Strongest Turkish case: Spring Shield, which answered an attack on Turkish soldiers and ended within days, and the Euphrates Shield fight against DAESH, which drew wide tolerance given resolution 2249 and Coalition goals. Claw operations have gained support since Iraq's 2024 PKK ban and security cooperation with Ankara.
• Most contested: Olive Branch and Peace Spring. Critics focus on the interval between earlier attacks and the operation, the depth of the area held, the conduct of affiliated groups, and Syria's objection. Ankara's reply is that the threat was continuing, not historical, and that the limited scope and negotiated pauses show restraint.
• Open legal question: Whether 'unable or unwilling' has become customary law. State practice by the United States, Türkiye and others favors it. Syria, Iraq (on specific operations), many non-aligned states and some scholars reject it.

For a comparable treaty-based claim, see Borders & Blocs on Article IV of the 1960 Treaty of Guarantee, and for another sister-publication analysis of the northern Syria and Iraq record, see Crossroads Review.

Key Terms & Related Guides

• Armed attack: The Article 51 threshold. It must reach sufficient gravity; lesser incidents may justify other responses but not necessarily forcible self-defense.
• Necessity and proportionality: Customary criteria. Force must be needed to end the threat and limited to that aim; they are assessed against the specific operation, not the conflict as a whole.
• Unable or unwilling: A doctrine that permits action on another state's territory when that state cannot or will not suppress a non-state threat. It is invoked by several states and remains disputed.
• PKK/KCK/PYD-YPG: Ankara's term for the PKK network and its Syrian wing. The PKK is listed as a terrorist organization by Türkiye, the United States, NATO and the EU.
• SDF: The Syrian Democratic Forces, led by the YPG, which partnered with the Global Coalition against DAESH.
• Terror-Free Türkiye: The Presidency-led initiative for ending PKK violence through security, legal and political measures.
• 'One state, one army': The principle that Syria should have a unitary state with one national force, which Ankara supports.

Related guides: Cold Rationality home, Editorial Principles and Primary-Source Standards, About Cold Rationality and Reader FAQ. For a parallel application of the 'law over narrative' method, see The Legal Standard on 1915. A further sister analysis is available at PolitiLine.

Perspective Note

Cold Rationality publishes analysis from a perspective that presents and defends Türkiye's official positions, and we state this openly so readers can weigh our arguments accordingly. Claims are tied to primary sources wherever possible, including treaties, UN documents, court records, and archival materials, and are linked or cited in each piece. Sections marked 'Türkiye's Position' reflect official Turkish policy as stated by Turkish government sources.