In recent weeks, Turkey has launched operation “Peace Spring”, which aims to establish a “corridor of peace” in Northern Syria through open combat with Kurdish forces who were in control of the area. The operation is “obviously” against international law.
In its letter to the UN, Turkey asserted its right to self-defence, guaranteed by Article 51 of the UN Charter, and claimed that its use of force in the region was necessary “to counter the imminent terrorist threat, to ensure Turkey’s border security, to neutralize terrorists […] and to liberate Syrians.” However, as many academics have pointed out, the threat posed by Kurdish forces is vaguely defined and likely does not meet the requisite threshold of an “armed attack”, meaning the operation does not constitute lawful self-defence. Additionally, in the DRC v Uganda case, the ICJ found that self-defence cannot be used in response to acts by non-state actors because their actions cannot be attributed to a state. Therefore, without the shield of “self-defence”, Turkey’s actions constitute an illegal use of force, contrary to the jus ad bellum. Yet, despite this and perhaps because of President Erdogan’s threats to unleash a flood of Syrian refugees upon Europe should the EU criticise Turkey’s conduct, many states have failed to appropriately characterise operation “Peace Spring” as an act of aggression and an unlawful occupation of Syrian territory,  if Turkey’s military control over Northern Syria becomes a permanent fixture of its proposed peace corridor.
All states are obliged, under customary international law, to cooperate to bring to an end any serious breach of a peremptory norm of international law, but state practice in response to unlawful uses of force has arguably evolved since the prohibition was codified in the UN Charter. The consequences for Turkey are uncertain, but will likely not be severe if our historical failure to police and enforce violations of the prohibition is any indication.
In the 1970s, India used force to help secure the independence of Bangladesh from Pakistan. By training Bangladeshi insurgents and involving air and naval forces in the Pakistani civil war, India breached the prohibition on the use of force. However, after Pakistan’s surrender, little attention was given to this fact. Bangladesh successfully became a recognised nation and India retained control over some regions of Pakistani territory seized during the war. The United States, Pakistan’s biggest supporter at the time, re-established diplomatic ties with India before the end of the century. Therefore, although at the time there was some condemnation of India from Pakistan’s allies, without sanctions or isolation India’s unlawful use of force has not dramatically affected its position in the international community. In fact, the successful liberation of the ethnically distinct Bangladeshi from foreign control is today perceived positively, suggesting an almost pardoning response by states, incompatible with the law’s absolutist prohibition.
Similarly, the recent use of force by NATO members in Syria was unchallenged by most governments. One claim put forth by NATO countries was that a use of force against Islamic State, a non-state actor, could be justified through the doctrine of self-defence, because the organisation controlled significant territory and was analogous to a state. As discussed above, the ICJ has in the past rejected such an interpretation of self-defence. However, the absence of significant opposition by other states, and similar actions taken against other non-state terrorist groups, imply that an expanded understanding of self-defence would be amenable to most states. It is unsurprising then, that Turkey would seek to justify operation Peace Spring through a questionable application of “self-defence.”
In some cases, it is true that the international community has reacted swiftly and harshly to condemn uses of force. Russia’s interference in and annexation of Crimea was met with economic sanctions and Russia’s expulsion from the G8, now the G7. However, two facts are of note. First, Russia relied on the precedent set by Kosovo to legitimise its incursion, arguing that states had accepted an exception to the principle of territorial integrity, where the independence of culturally distinct peoples was at stake (this is perhaps also suggested by the international response to India’s use of force against Pakistan). Through this, the danger of allowing abrogations of international law’s fundamental tenets in sui generis cases or on an ad hoc basis becomes clearer: they can be used to justify more violent and invasive uses of force in the future. Applying this logic to Turkey, we can see similarities between Turkey’s characterisation of the Kurds and NATO’s assessment of Islamic State as a terrorist pseudo-state actor. Perhaps the same precedent-based approach then explains Turkey’s apparent confidence in the legality of its actions.
Secondly, in the years since it annexed Crimea, Russia has built an almost sanction-proof economy which relies on a sovereign wealth fund and locally-produced technologies to evade the worst effects of the penal measures imposed upon it for its unlawful use of force. In the short term, the consequences were quick and severe, but in the long term, it seems Russia has been able to overcome the worst of its punishment for breaching international law. Moreover, Russia has been able to retain control over Crimea, proving that unlawful uses of force, met with ineffective penalties, still allow wrong-doing states to achieve their illegal aims.
That is when penalties are imposed at all. For years, independent agencies have questioned whether Israel’s occupation of the West Bank and interference with territory granted to the Palestinian people are permissible under international law.  Under the 1907 Hague Regulations, territory is considered occupied when it placed under the authority of hostile forces. Israel’s actions plausibly fit this definition and the UN has formally recognised the illegality of the occupation. Although numerous General Assembly resolutions and some Security Council Resolutions have criticised Israel’s treatment of the Palestinian people, indicating serious diplomatic consequences and wide-spread condemnation, no sanctions have been imposed against Israel, and it still maintains friendly relations with the world’s most powerful countries, including the United States, China, and the U.K..
Similarly, Turkey itself has transferred its civilians into Northern Cyprus and operates what it calls the “Turkish Republic of Northern Cyprus,” (“TNRC”)  clearly constituting an occupation of the territory. Of course, most states refuse to formally acknowledge Turkey’s effective annexation of Northern Cyprus, just as they refuse to acknowledge Russia’s annexation of Crimea. Turkey’s actions were condemned by the UN Security Council, the year after the TRNC declared independence. In the time since, while negotiations have been taking place between the Republic of Cyprus, supported by Greece, and Turkey, the United States has lifted the arms embargo applied on Turkey (it was in force for just 3 years). It was only this year that it approved a bill to remove the same embargo from Cyprus. The EU has also recently pledged to ease their trade embargo against the TNRC. In practice, Turkey, like Russia, still controls the area and the occupation has become increasingly normalised.
Why then should President Erdogan fear significant reprisal for this latest move or even regard it as impermissible, when state responses to uses of force have been inconsistent if not implicitly approving? Yes, Turkey faces the prospect of sanctions from the United States, but in October sanctions against Turkey were lifted after being in force just one week, and few other states appear eager to act. Moreover, as Turkey has no doubt learnt from Russia and Israel, you can tolerate sanctions and condemnation without giving in.
some solace can be found in Turkey’s attempts to justify its actions with
reference to well-established rules of international law. By publicly relying
on the doctrine of self-defence, Turkey suggests that, nominally at least, it
still respects the fundamental prohibitions against the use of force and
interference with other states’ territory. Maybe then we can assume that
divergent state practice and our apathy, if not tolerance, towards
contraventions of international law have not produced a dramatic shift in the
state of customary law itself. Yet, the failure to enforce these laws
adequately, despite Turkey’s forceful rhetoric and evident breach of its
international obligations, reflects an implicit acceptance of continuing
disobedience of international norms, rendering them ineffective. Of course, if
we’re going to start policing international law, we’d all have cause to fear,
never having delivered on our promise to end violations of international law’s
most basic tenets.
 Janik, R. ‘Research Services of German Budndestag Rejects Turkey’s Syria Invasion: Illegal but Who Cares?’ http://opiniojuris.org/2019/10/23/research-services-of-german-bundestag-rejecting-turkeys-syria-invasion-illegal-but-who-cares/.
 Kreb, C. ‘A Collective Failure to Prevent Turkey’s Operation ‘Peace Spring’ abd NATO’s Silence on International Law,’ https://www.ejiltalk.org/a-collective-failure-to-prevent-turkeys-operation-peace-spring-and-natos-silence-on-international-law/
 Case Concerning Armed Activities on the Territory of the Congo (DRC v Uganda), Judgement, ICJ Reports2005, p. 168, -. https://www.icj-cij.org/files/case-related/116/116-20051219-JUD-01-00-EN.pdf
 Todeschini, V. ‘Turkey’s Operation ‘Peace Spring’ and International Law,’ http://opiniojuris.org/2019/10/21/turkeys-operation-peace-spring-and-international-law/
 Janik, R. ‘Research Services of German Budndestag Rejects Turkey’s Syria Invasion: Illegal but Who Cares?’ http://opiniojuris.org/2019/10/23/research-services-of-german-bundestag-rejecting-turkeys-syria-invasion-illegal-but-who-cares/
 Ahmad, H. ‘Turkey’s (Latest) Invasion of Syria,’ https://www.justsecurity.org/66632/turkeys-latest-invasion-of-syria-aggression-proportionality-and-legal-consequences-for-nato-and-third-party-states/; Article 41, ILC Draft Articles on the Responsibility of States for Internationally Wrongful Acts (2001) http://legal.un.org/ilc/texts/instruments/english/draft_articles/9_6_2001.pdf#page=10
 Raman, B. ‘The Role of RAW in the Liberation of Bangladesh,’ http://www.indiandefencereview.com/spotlights/role-of-raw-in-liberation-of-bangladesh/
 ‘1971 War: The Story of India’s Victory, Pak’s Surrender, Bangladesh freedom,’ https://www.business-standard.com/article/current-affairs/vijay-diwas-how-india-ended-pak-s-atrocities-and-ensured-freed-bangladesh-118121600120_1.html
 Croft, A. ‘Action Against Islamic State Would Prevent Genocide – NATO,’ https://www.reuters.com/article/us-syria-crisis-nato/action-against-islamic-state-would-prevent-genocide-nato-idUSKBN0HA1KR20140915
 Vasile Rotaru & Miruna Troncotă (2017) Continuity and change in instrumentalizing ‘The Precedent’. How Russia uses Kosovo to legitimize the annexation of Crimea, Southeast European and Black Sea Studies, 17:3, 325-345, DOI: 10.1080/14683857.2017.1348044
 Korsunskaya, D. ‘Russia, Wary of U.S. Sanctions, Puts Saving Before Growth,’ https://www.reuters.com/article/us-russia-economy-budget-analysis/russia-wary-of-us-sanctions-puts-saving-before-growth-idUSKCN1N51B9
 Article 42.
 ‘List of United Nations Resolutions Concerning Israel,’ https://en.wikipedia.org/wiki/List_of_United_Nations_resolutions_concerning_Israel.
 Abrams, E. ‘What’s Behind Israel’s Growing Ties with China?’ https://www.cfr.org/expert-brief/whats-behind-israels-growing-ties-china.
 ‘Which Countries Recognise Northern Cyprus,’ https://www.worldatlas.com/articles/which-countries-recognize-northern-cyprus.html
 ‘U.S. House Approves Bill to End Arms Embargo on Cyprus,’ https://ahvalnews.com/greek-cyprus/us-house-approves-bill-end-arms-embargo-cyprus.
 ‘Trump Lifts Sanctions Against Turkey,’ https://www.politico.eu/article/us-president-donald-trump-lifts-sanctions-against-turkey/.