Law | Middle East | Tim Hagemann

Limitless Self Defence? Turkey's invasion of Northern Syria in light of Art. 51 UN Charter

In this article, Tim Hagemann will analyze the general preconditions for self-defense in international law and their applicability to the Turkish military operations in Northern Syria in light of the perception of self-defense in the last two decades.


Just a few days after the Trump administration withdrew US troops from Northern Syria, on 9th October 2019, the Turkish Armed Forces and its Syrian rebel allies crossed the borders to the territory hold by the Kurdish dominated Syrian Democratic Forces. The declared aim of “Operation Peace Spring”, the third incursion into Syria after the Dscharablus (2016) and Afrin (2018) campaigns, is to establish a 30 km (20 mi) “safe zone” between the Turkish border and the Kurdish forces accused of collaborating with terrorists of the PKK (Kurdish Workers’ Party). Predominantly, this operation serves the security interests of the Turkish Republic fearing the spill over of Kurdish insurgents into its territory and the instigation of rebellion among the Kurdish population of the south-east, but double-acts to establish a resettlement area for Syrian refugees in Turkey. Politically, the Erdogan administration profits in several ways from the operation, both by shifting the media attention from the low performing economy as well as solving the refugee problem that just recently caused painful losses to the AKP in important regional elections such as in Istanbul or Ankara.

While the political motives for the invasion seem rather clear, its legal justification is highly disputed. According to Hulusi Akar, Turkish Minister of Defence in the AKP government, Turkey simply exercised its internationally recognized right to self-defence against terrorists by occupying Syrian territory.[1] However, this would heavily collide with the traditional understanding of self-defence in international law and practice, that is particularly sceptic on its applicability to fighting non-state actors in foreign territory. Nevertheless, with the last two decades seeing a rise of cross-border terrorism in unwilling or unable host states and the international community’s responses from Afghanistan to Iraq and Syria, the question must be posed, if international customary law has changed in a way that would allow the Turkish invasion into Northern Syria. Accordingly, this article will analyze the general preconditions for self-defence under international law, the categorization of the Turkish military operation and finally, if there was a shift in the perception of self-defence.

The use of force in international law

According to Art. 2 (4) UN Charter, all members of the United Nations are bound to:

“refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State , or in any other manner inconsistent with the purposes of the United Nations.”

This general provision is limited by Chapter VII UN Charter, that authorizes the UN Security Council to take coercive measures including the authorization of military operations in case of a threat or breach of peace or an act of aggression. While certainly giving the UN Security Council priority in dealing with violations of international peace, these provisions were not intended to preclude states from exercising their right to self-defence against foreign aggression as long as the Security Council did not take action.[2]

Accordingly, Art. 51 UN Charter states, that:

“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.”

The language as well as the travaux préparatoires of Art. 51 UN Charter demonstrate the intention of the member states to adhere to the customary right to self-defence in international law in the limits stipulated by Art. 51 UN Charter instead of creating a UN specific right to self-defence.[3] This leads to two conclusions: First, that the preconditions as set out by international customary law apply to acts of self-defence and second – more interestingly – that these preconditions are not set in stone, but subject to the interpretation and practice of the international community.

Traditionally, international customary law set out two conditions for the lawful exercise of the use of force in the context of self-defence:[4]

1.     The use of force must be conducted in response to an imminent armed attack

2.     The defence actions must be necessary and proportionate to the armed attack

Additionally, Art. 51 UN Charter obliges the defending state to report any taken measure to the UN Security Council and refrain from any actions as soon as the UN Security Council intervenes.

Application to Operation Peace Spring

While Turkey did indeed report its measures and alleged justification to the UN Security Council, it is debatable, if Turkey complied with all requirements for lawfully exercising its right to self-defence in its traditional interpretation. First, Operation Peace Spring must have been exercised in response to an imminent armed attack by a feasible (non-state) actor and second, the military invasion of Northern Syria to implement a “safe zone” must be necessary and proportionate to defend Turkey from this armed attack.

Armed attack

In its Nicaragua Judgment, the International Court of Justice stated that not every exercise of the use of force amounts to an armed attack as understood in Art. 51 UN Charter, but must reach a particular scale and effect that exceeds that of a “mere frontier incident”.[5] In other words, not every violation of Art. 2 (4) UN Charter entitles the impaired state to counter-violence in accordance with Art. 51 UN Charter, but only those considered the “most grave forms of the use of force”[6]. If the violation of Art. 2 (4) UN Charter does not exceed the critical threshold of an “armed attack” in scale and effect, the response of the impaired state must not be a violent act of self-defence, but merely non-violent – especially economic – countermeasures.[7]

As for Operation Peace Spring, the Turkish foreign minister Mevlut Cavusoglu claimed that the threat of alleged PKK terrorists spilling over into Turkish territory as well as permanent violent attacks by YPG rebels would constitute such a situation in which Turkey was entitled to self-defence in accordance with Art. 51 UN Charter and the relating UNSC resolutions.[8] However, the Turkish government failed to produce any convincing evidence relating to the alleged permanent attacks by YPG forces – or more precisely – it did not even try to produce any evidence.[9] In fact, the last major violent attack of Kurdish separatists against Turkish citizens that caught domestic and international media coverage dates back to 2016. But even in the event that the YPG was responsible for isolated terror attacks or occasional border skirmishes, this would not amount to the necessary gravity of use of force as outlined in Nicaragua. Although there is a debate, if isolated events that are conducted as part of a terror strategy must be seen as one coherent attack (accumulation of events doctrine) – there is, however, insufficient state practice and unanimous condemnation of this approach by the Security Council to justify a legal exception.[10]

Conclusion: There is no evidence of a use of force by SDF rebels that would exceed the scale and effect of an armed attack.


The wording of Art. 51 UN Charter “if an armed attack occurs” points to a very restrictive interpretation, where self-defence is only feasible when an armed attack is already in motion.[11] However, international customary law has accepted certain exceptions from this principle as early as in the Caroline Affair of 1837: When the defending state is able to produce evidence that an attack was so imminent and overwhelming that the use of force was the only available choice to prevent the attack, the right to self-defence might be exercised by a preventive strike.[12] Accordingly, interceptive strikes such as those conducted by the IDF in the Six-Days War have been recognized as legitimate self-defence by the majority of the international community[13], while the IDF’s preemptive strikes against an Iraqi nuclear power plant under construction in 1981 have been condemned by the UN Security Council.[14]

The last terrorist attack attributable to Kurdish rebels – not necessarily to the SDF itself – has been in 2016. Since then, neither a major violent incident has been reported in domestic or international media nor did Turkey produce any evidence of an imminent threat by the SDF. Instead, the Turkish government resorted to the accumulation of events doctrine that has not been able to find its way into international customary law yet.

Conclusion: The claimed armed attack by the SDF was not imminent.

The SDF as feasible target

It is necessary to keep in mind that Operation Peace Spring – like the Dscharablus and Afrin campaigns before – is not violating the right of territorial sovereignty of the Kurdish de facto regime Rojava from which the SDF operates, but of the Arab Republic of Syria, that – de jure – is still overlord over the targeted regions. The question, if self-defence can be exercised against non-state actors is as old as it is controversial. Although the ICJ has been cautious on the matter in the past[15], it is fair to say, that at the latest with UN Security Council Resolutions 1368 and 1371 responding to the 9/11 terror attacks, self-defence against an armed attack conducted by non-state actors is possible, if certain preconditions apply.[16] The Afghanistan Criteria are satisfied when the state, whose territorial integrity is impaired, is substantially involved with the attacked non-state actor. That means, the state must support the non-state actor (esp. training and armament) and maintain a close political relationship.[17] As until recently, the SDF maintained a fragile peace with the Assad regime, envisioned an independent state on Northern Syria and managed their occupied territory autonomously, there is hardly any kind of involvement between the two.

As autonomous overlords over great parts of North-Western Syria and the border region to Turkey, the SDF built a de-facto regime which effectively and independently exercises power over its population. De-facto regimes, however, are themselves obligated by Art. 2 (4) UN Charter. Accordingly, in the interest of stable international order, they must be a viable target for the exercise of self-defence.[18]

Conclusion: In general, the SDF is a viable target for the exercise of self-defence by Turkey.


Even if the threat by the SDF forces would amount to an armed attack, it is well established in international law, that an act of self-defence is only feasible, when it is necessary to respond to an armed attack.[19] Accordingly, the invasion of Northern Syria must be an appropriate measure to respond to an alleged terrorist or insurgence threat by the SDF against Turkey.

In the context of Operation Peace Spring – and given that a terrorist threat by the SDF actually exists – Turkey could not have achieved the pacification of its border region without resort to the use of force, which – considering the official objectives of that mission – would not exceed an unreasonable degree of violence under Art. 51 UN Charter.[20] It will be crucial, however, to observe the future course of the military operation and if Turkey sticks to its objectives.[21]

Clearly, it would have been preferable that Turkey would have negotiated first with Syria’s central government and achieve consent regarding its military intervention or to give it the opportunity to sort out this threat for Turkey first. As Syria’s army, however, did not seem to be in any position to prevent the SDF’s conduct in the first place, a preceding negotiation would have been a pointless formalism anyways.

Conclusion: Given that the terrorist threat of the SDF amounted to an armed attack, the military invasion of Northern Syria and the establishment of a “safe zone” would probably pass the threshold of necessity.


Even in the event that one accepts the Turkish argument that the SDF engaged in an armed attack against the Turkish Republic and at the same time, constitutes a feasible target, the ICJ made it clear that any exercise of self-defence must strictly adhere to the principle of proportionality.[22] In this capacity, the defending state possesses no political scope of appreciation.[23] While the response to an armed attack does not have to be strictly symmetric to the initial attack, it must nonetheless be proportional in regard to its nature and intensity, armament, geopolitical framework and the expected damages on the side of the attacker.[24] Especially, the defending state is not restrained to fighting an attacker on its own territory if that seems necessary to effectively repel the armed attack.[25]

Even when following the Turkish interpretation that the SDF’s conduct so far constitutes an armed attack, the indefinite occupation of Northern Syria must hence be characterized as unproportionate. In regard to the annexation of the West Bank and the occupation of the Golan Heights by Israel following its lawful exercise of self-defence, the UN Security Council and the General Assembly have repeatedly expressed, that even if the initial use of force is lawful, the annexation and prolonged occupation of captured territory constitutes a violation of international law.[26] Accordingly, the establishment of a 30 km deep “safe zone” in Syrian territory controlled by Turkey and its allies exceeds the threshold of proportionality, first, because the resettlement of Syrian refugees under the control of Turkey deprives the Kurds of the control over this territory indefinitely and second, because the occupation of a territory of that proportion is not necessary to prevent future attacks by the SDF and rather seems to serve ulterior motives.

Conclusion: The establishment of a “safe zone” is an unproportionate exercise of self-defence.

Are we entering a period of limitless self-defence?

As outlined above, the traditional understanding of the right of self-defence does not legitimate the invasion of Northern Syria by Turkey and its allies. However, it is at least debatable, if the interventions in the armed conflict in Syria have drastically changed the conditions under which self-defence is lawful against non-state actors in international customary law.[27] Among others, the USA, the UK, Turkey and France have invoked individual self-defence against Daesh (the so-called Islamic State) as justification for participation in the Syrian Civil War.[28] In the same vein, Turkey cited Self-defence against terrorist activities (and the Adana agreement[29] of 1998), as justification for Operation Peace Spring when its representative informed the Security Council of the events.[30] Although being protested and called a violation of international law by many scholars, the question emerged if the fight against Daesh and the accompanying reasoning by the intervening states has created a precedence in international customary law, expanding the right to self-defence against terrorism.[31] This would dramatically reduce the traditional requirements of the use of force – instead of an imminent armed attack and the strict adherence to proportionality, a mere abstract threat would arguably be sufficient to invoke self-defence. In order to constitute a shift in international customary law, the reduced conditions must be reflected in the concerned parties’ state practice and opinio juris.[32]

In fact, many states have invoked a right to self-defence before the Security Council when fighting non-state actors in foreign territory. E.g. Iran and Turkey have justified its operations in Iraqi territory to fight Kurdish rebels as self-defence since the 90s.[33] More interestingly, aside from France and Belgium, that were the target of terrorist acts arguably amounting to the gravity of an armed attack in the sense of Art. 51 UN Charter, the UK, the USA and Saudi-Arabia (as well as in Yemen) cited individual and Germany, Australia and Canada collective self-defence as justification for their military involvement in Syria. Legally, this is supported by the argument, that states threatened by violent acts of terrorism emanating from another state’s territory must have a right to effectively defend itself when the harboring state either does not have the will or the capacity to suppress such acts (unwilling or unable doctrine), which was supposedly supported by UNSC Resolution 2249 in 2015.[34]

However, both the grounds for the legal argument for the unwilling and unable doctrine and the notion of its unanimous support must be severely challenged in light of contemporary international law. Accordingly, the unwilling or unable doctrine’s legal reasoning and its consequences for the international peace order have to be critically evaluated. Legally, the ICJ has always been very reluctant of expanding the right to self-defence to non-state actors at all.[35] This interpretation can also not be challenged by the UNSC Resolution 2249. While the Resolution acknowledged the threat emanating from Daesh, in sharp contrast to preceding resolutions, the UNSC deliberately did not use the term “self-defence”.[36] Moreover, even the Anti-IS coalition members themselves did not unanimously endorse the doctrine (e.g. Germany,

Belgium, Norway and the Arab states). Furthermore, the reaction by the vast majority of member states in relation to the doctrine was strictly negative. E.g., in a joint declaration, China and Russia strongly opposed the invocation of self-defence in relation to non-state actors. [37] It is important to note, that the reaction of the latter does not constitute a contradiction for their involvement in the Syrian Civil War as they were invited by the Assad government to participate on the government’s side. Likewise, the Non-Aligned Movement affirmed their unshaken view that Art. 51 UN Charter shall not be extended beyond its traditional understanding.[38] Finally, such an expansive understanding of self-defence could severely endanger the international peace order as it works as a slippery slope for the unilateral invocation of the use of force. While the UN Charter’s idea of collective security already faces wide-spread disregard and self-defence has become the dominant justification of the use of force instead of measures under Chapter VII as intended, an expansion of Art. 51’s applicability could further deteriorate multilateralism in international relations.[39]

Accordingly, the notion, that international customary law has changed in recent years to a degree, where states could invoke self-defence against non-state actors to lawfully operate militarily on foreign ground when the state is unwilling or unable to fight terrorist activities emanating from its territory has to be severely dismissed. Neither does contemporary international offer a basis for such an assumption nor does the rejection of the principle by the vast majority of the international community sufficiently impact international customary law to justify self-defence. Although the interests of states to defend their territorial integrity and population against non-state aggressors must be recognized, in the interest of the global peace order, this should not be construed to serve as an excuse for military unilateralism. In effect, a state targeted by foreign terrorists should first seek an understanding with the harboring state before unilaterally violating its territorial sovereignty. If the state is unwilling or unable to fight the terrorist threat, support by the UNSC is not in sight and the harboring state rejects military assistance by affected states, it would still be preferential to invoke (collective) counter measures such as sanctions to force the harboring state’s consent to a targeted police operation as some scholars correctly suggested.[40]

In relation to Turkey, the international community should thus call Operation Peace Spring as what it is: A drastic violation of the prohibition of the use of force under Art. 2 (4) UN Charter and a move that serves domestic political interests by severely disregarding the very foundations of the international order. Especially its NATO partners should remind Turkey of Art. 1 of the North Atlantic Treaty, where it reads, that every NATO member must:

“settle any international dispute in which they may be involved by peaceful means in such a manner that international peace and security and justice are not endangered, and to refrain in their international relations from the threat or use of force in any manner inconsistent with the purposes of the United Nations.”

[2] ICJ, Military and Paramilitary Activities in and against Nicaragua, ICJ Reports 1986, 14, para. 176.

[3] Greenwood, Christopher: Self Defence, Max Planck Encyclopedias of International Law [MPIL], para. 3.

[4] Greenwood, Christopher: Self Defence, Max Planck Encyclopedias of International Law [MPIL], para. 8.

[5] ICJ, Military and Paramilitary Activities in and against Nicaragua, ICJ Reports 1986, 14, para. 195.

[6] ICJ, Military and Paramilitary Activities in and against Nicaragua, ICJ Reports 1986, 14, para. 191.

[7] Greenwood, Christopher: Self Defence, Max Planck Encyclopedias of International Law [MPIL], para. 12.

[8] E.g. via his official Twitter account:

[9] Schwarz, Alexander: „Regelbasierte Weltordnung“ unter Beschuss,

[10] UNSC Res. 490; UNSC Res 501; UNSC Res 509.

[11] v. Arnauld, Andreas: Völkerrecht, 2nd edition, para. 1065.

[12] v. Arnauld, Andreas: Völkerrecht, 2nd edition, para. 1065.

[13] v. Arnauld, Andreas: Völkerrecht, 2nd edition, para. 1066.

[14] S/RES/487.

[15] Cf. Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, ICJ Reports 2004, 194, para. 139.

[16] Cf. S/RES/1368 and S/RES/1371 (2001).

[17] v. Arnauld, Andreas: Völkerrecht, 2nd edition, para. 1091.

[18] v. Arnauld, Andreas: Völkerrecht, 2nd edition, paras. 67, 1097.

[19] ICJ, Military and Paramilitary Activities in and against Nicaragua, ICJ Reports 1986, 14, para. 176.

[20] Schwarz, Alexander: „Regelbasierte Weltordnung“ unter Beschuss,

[21] Please note, that the necessity of violence only refers to Turkey’s right to self-defence; If the alleged killing of prisoners or the targeted bombing of civilians was “necessary” is a question of Humanitarian Law.

[22] ICJ, Advisory opinion on the Legality of the Threat or Use of Nuclear Weapons, 226, para. 41.

[23] ICJ, Oil Platforms, 161, para. 73.

[24] v. Arnauld, Andreas: Völkerrecht, 2nd edition, para. 1069.

[25] Schwarz, Alexander: „Regelbasierte Weltordnung“ unter Beschuss,

[26] S/RES/252; S/RES/267; S/RES/271; S/RES/298; S/RES/465; S/RES/476; S/RES/497; A/67/L.24.

[27] Cf. MPIL Research Paper Series No. 2017-07, p. 3.

[28] USA (23.9.2014) S/2014/695; UK (25.11.2015), S/2014/851 and (7.9.2015) S/2015/688; Turkey (24.7.2015), S/2015/563; France (8.9.2015) S/2015/745.

[29] A Turkish-Syrian agreement concerning Syria’s cessation of support for the PKK in the Turkish border region. However, the treaty only allows Turkey to pursue PKK forces 5 km into Syrian territory and sets a very limited timespan for the removal of Turkish forces after the pursuit. As this would severely contrast the operation’s objective of creating a “safe zone”, its importance for Turkey’s legal argument can be considered marginal.

[30] Cf. S/2019/804.

[31] So - inter alia - Prof. Stefan Talmon of University of Bonn,

[32] ICJ, North Sea Continental Shelf, 1969, pp. 3, 43.

[33] Cf. S/1996/479; S/25843.

[34] E.g. MPIL Research Paper Series No. 2017-07, pp. 49 ff; Cf. S/RES/2249.

[35] E.g. in: ICJ, Congo, 2005, 168, para. 147.

[36] MPIL Research Paper Series No. 2017-07, p. 18.

[37] Chinese-Russian letter to the UN Secretary General of 8.7.2016 contained the Declaration of the Russian Federation and the People’s Republic of China on the Promotion of International Law, signed in Beijing on 25.6.2016 by the two states’ Ministers of Foreign Affairs (UN Doc. S/2016/600), para. 7.

[38] S/PV.7621, 15.2.2016, at 33 et seq.

[39] MPIL Research Paper Series No. 2017-07, p. 19.

[40] v. Arnauld, Andreas: Völkerrecht, 2nd edition, para. 1095.


Tim Hagemann

Tim Hagemann studied International Law, Economics and Chinese at the Universities in Tuebingen, Beijing, Aix-en-Provence and Maastricht. He holds a Master's degree in European and International Law from Aix-Marseille University, passed his first legal State Examination in Germany with a focus on public international and economic law and currently pursues an additional Master's degree in International Trade and Investment Law at Maastricht University. Professionally, he works as research associate for a renowned law firm and authors books relating to international business and economic law.