My research interests relate to foundational questions of international law, especially its constitutionalisation and de-constitutionalisation in the context of global governance, the status of humans in international law, global animal law, law surrounding armed conflict, and the histories of international law.

• In the 1990s, the activities of international organisations, international law-making and also the extraterritorial repercussions of state action became more intense and altogether created a regulatory web usually called “global governance”. This has led to an erosion of the constitutional law of nation states. This erosion was to some extent compensated for by the emergence of a body of global constitutional law (“compensatory constitutionalism”). The relevant norms and institutions are found in international law in interaction with national law; they are “global” or “transnational”. The functions of global constitutional law were and are the constitution and limitation of international and transnational authority. In addition, the idea of global constitutionalism has been to identify and promote the constitutionalist principles of the rule of law, human rights and democracy also in the global sphere, particularly in the 1990s.
  (“Compensatory Constitutionalism: The Function and Potential of Fundamental International Norms and Structures”, Leiden Journal of International Law 19 (2006), 579-610; “The Globalisation of State Constitutions, Chapter 10”, in: Janne Nijman/André Nollkaemper (eds.), New Perspectives on the Divide between National and International Law, OUP 2007,251-308; The Constitutionalization of International Law, OUP 2009/2011 (with Jan Klabbers, Geir Ulfstein); “The Merits of Global Constitutionalism”, Indiana Journal of Global Legal Studies 16 (2009), 397-411; “Are we Moving towards Constitutionalisation of the World Community?”, in: Antonio Cassese (ed.), Realizing Utopia: The Future of International Law, OUP 2012, 118-135; “Fragmentation and Constitutionalisation”, in: Anne Orford/Florian Hoffmann (eds.), The Oxford Handbook of the Theory of International Law (OUP 2016), 1011-1031; Takao Suami, Anne Peters, Dimitri Vanoverbeke and Mattias Kumm (eds.), Global Constitutionalism from European and East Asian Perspectives (CUP 2018)).
• In the era of globalisation fatigue, a number of trends point towards the emergence of a “more social” international law in which a cross-border social responsibility for individuals is legally acknowledged. By absorbing the global social question, global constitutionalism can mitigate its neo-liberal tilt, and would be rescued from being reduced to a project to simply augment the power of capital. (“Global Constitutionalism: The Social Dimension”, in: Global Constitutionalism from European and East Asian Perspectives (CUP 2018), 277-350). Across the globe, constitutionalist principles have been consolided and implementedonly unevenly. Reasons are a lack of fit to diverse regional economic, political and cultural conditions in the world, a loss of credibility following overstretching and breaches of international law by states in the West, and finally shifts of economic, political and discursive power towards post-colonial states in the Global South. Particularly since the illegal Iraq War led by the US (2003) and the global financial crisis (2008), transnational and international de-constitutionalisation movements can be observed. (“Against a Deconstitutionalisation of International Law in Times of Populism, Pandemic, and War”, Journal of Constitutional Justice 9 (2022), 135-192; The Russian Invasion in Ukraine: An Anti-Constitutional Moment in International Law? - The Skubiszewski Lecture 2023 - Poznań Journal of Law, Economics and Sociology 2024). These trends interact with erosions of democracy and the rule of law in numerous states in all regions of the world. Persistent human rights violations by states and economic actors, largely unsanctioned massive violations of ius contra bellum (Russian invasion of Ukraine in 2022) and ius in bello (including in the Gaza war since 7 October 2023) as well as cynical assertions of international law and “double standards” call into question the legitimacy and effectiveness of the international legal order as a whole. (Anne Peters and Anna Petrig, Völkerrecht: Allgemeiner Teil, 6th ed. (Schulthess/C.F. Müller 2023), Chapter 1).
• International law scholars can meet the current challenges of the ecological catastrophe, massive transnational material inequality and ideational polarisation by adapting fundamental principles and demanding a consistent application of the law. This includes the development of fair procedures. The new techniques which notably courts and tribunals have developed in a ‘spirit of systemic harmonisation’ (Al-Dulimi), in order to coordinate the various subfields of international law can be seen as a form of procedural constitutionalisation. (“The Refinement of International Law: From Fragmentation to Regime Interaction and Politicization”, International Journal of Constitutional Law 15 (2017), 671-704).
• The rise of due diligence reflects the current trend of strengthening procedural rules (as opposed to substantive rules) in international law. Due diligence forces states to develop risk management procedures and contributes to holding states accountable. On the other hand, the focus on mere due diligence risks diluting legal obligations and thereby weakening the international legal order. (Due Diligence in the International Legal Order (ed. with Heike Krieger and Leonhard Kreuzer) (OUP 2020); “Due Diligence: the Risky Risk Management Tool in International Law”, Cambridge International Law Journal 9 (2020), 121-136 (together with Heike Krieger and Leonhard Kreuzer).
• The principle of proportionality is a global constitutional principle. (“A Plea for Proportionality: A Reply to Yun-chien Chang and Xin Dai”, International Journal of Constitutional Law (2021), 1134-1145; “Proportionality as a Global Constitutional Principle”, in: Anthony F. Lang/Antje Wiener (eds.), Handbook on Global Constitutionalism (Edward Elgar Publishing 2nd ed. 2023), 346-362).
• In the era of populism, nationalism and new authoritarianism, international legal rules risk being superseded by the national law of individual states. Actors in national legal systems apply techniques that are intended to avoid and resolve conflicts of norms. Techniques that are “friendly to international law” include a procedural obligation to take international law into account, the interpretation of domestic law in conformity with international law, and the legal presumption that rules or institutions stemming from different legal orders are equivalent. These techniques should be expanded and refined in order to keep the global network of norms functional. (“Supremacy Lost: International Law Meets Domestic Constitutional Law”, Vienna Online Journal on International Constitutional Law 3 (2009), 170-198; “Rechtsordnungen und Konstitutionalisierung: Zur Neubestimmung der Verhältnisse”, Zeitschrift für öffentliches Recht 65 (2010), 3-63; Remedies against Immunity? Reconciling International and Domestic Law after the Italian Constitutional Court's Sentenza 238/2014 (Springer 2021) (ed. with Valentina Volpe and Stefano Battini); “Business and Human Rights: Towards a ‘Smart Mix’ of Regulation and Enforcement", Zeitschrift für ausländisches öffentliches Recht und Völkerrecht 83 (2023), 415-459 (with Sabine Gless/Chris Thomale/ Marc-Philippe Weller); “The American Law Institute's Restatement of the Law: Bastion, Bridge and Behemoth”, European Journal of International Law 32 (2022), 1377-1397; “Völkerrechtsfreundlichkeit ─ mehr als ein Lippenbekenntnis”, Zeitschrift für ausländisches öffentliches Recht und Völkerrecht 83 (2023), 587-608; ‘’Füg‘ dich, meine Schöne’: Plädoyer für ein feministisches Foreign Relations Law”, Zeitschrift für ausländisches öffentliches Recht und Völkerrecht 84 (2024), 7-22).

• International organisations continue to be central actors in global governance. The interpretation of the founding documents of international organisations as constitutions sought to justify the consolidation and even expansions of powers of the organisations after 1945 and – in a countermove – to secure their accountability after 1990. (“Das Gründungsdokument internationaler Organisationen als Verfassungsvertrag”, Zeitschrift für öffentliches Recht 68 (2013), 1-57; “International Organizations and International Law”, in: Jacob Katz Cogan/Ian Hurd/Ian Johnstone (eds.), The Oxford Handbook of International Organizations (OUP 2016), 33-59; “Constitutional Theories of International Organisations: Beyond the West”, Chinese Journal of International Law 4 (2021), 649-69; “International Organisations”, in: Richard Bellamy/Jeff King (eds.), Cambridge Handbook of Constitutional Theory (CUP 2024)).
• Bodies such as the Security Council (or its members) are increasingly subjected to legal limitations, which are, however, constantly being questioned and re-discussed. (“The Security Council's Responsibility to Protect”, International Organisations Law Review 8 (2011), 1-40; “The War in Ukraine and the Curtailment of the Veto in the Security Council”, Revue Européenne du Droit 4 (2023), 87-93; “The War in Ukraine and Legal Limitations on Russian Vetoes”, Journal on the Use of Force and International Law 10 (2023), 162-172; “Art. 24” and “Art. 25”, in: Bruno Simma/Daniel-Erasmus Khan/Georg Nolte/Andreas Paulus (eds.), The Charter of the United Nations: A Commentary on the UN Charter, 4th ed. (OUP 2024).
• Institutions without international legal personality, provided they have a minimum degree of autonomy, are ultimately “global” organisations that must in turn meet legitimacy requirements. (The Legal Framework of the OSCE (co-ed. with Mateja Steinbrück Platise/Carolyn Moser (CUP 2019); “The Concept of International Organization” (with Angelo Golia), in: Jan Klabbers (ed.), The Cambridge Companion to International Organizations Law (CUP 2022), 25-49).
• The participation of international organisations in constitutional processes in member states is lawful and legitimate, but often not very effective. In order to increase acceptance and thus have a more sustainable impact, such constitutional assistance must be adapted to the post-colonial constellation. This includes respect for local rule-of-law cultures flowing from non-European constitutional thought and the inclusion of a much deeper social agenda with a global ambition. (“International Organizations as Constitution-Shapers: Lawful but Sometimes Illegitimate, and Often Futile”, UC Irvine Journal of International, Transnational, and Comparative Law 8 (2023), 61-106; “International Organizations as Constitution-shapers: Promoting or Undermining a Transnational Rule of Law?”, in: Gregory Shaffer/Wayne Sandholtz (eds.), The Rule of Law under Pressure: A Transnational Challenge (CUP 2024).
• Non-state actors in all three sectors (inter-governmental, civil society, and business) have established themselves as legitimate and effective standard setters, whose relevance is currently not diminishing despite the erosion of the international legal order. (Non-State Actors as Standard Setters (CUP 2009) (coed. with Lucy Koechlin/Till Förster/Gretta Fenner Zinkernagel)).
• Fundamental legal norms of the EU can and should be qualified as a constitution of the EU (irrespective of the lack of a formal constitutional document). This European constitution acquires its legitimacy mainly through the test of time, by its output (i.e. legal and political outcomes in the European public interest), less through its genesis and through input by the European citizens in elections and referendums. (Elements of a Theory of the Constitution of Europe (Duncker & Humblot 2001).

• International law suffers from a democratic deficit that arises from the lack of a world parliament, from the absence of democratic law-making procedures, and missing options for direct participation of citizens in international decision-making. International law and global governance can be made more democratic with various strategies, even if a world parliament seems unfeasible. Such a democratisation is necessary in order to complement the indirect democratic basis that international law and governance enjoys through the involvement of national parliaments and governments (“dual democracy”). (“Dual Democracy”, in: Klabbers/Peters/Ulfstein, Constitutionalization 2009/2011, 263-341).
• A general principle of transparency has emerged as a foundational principle across all special fields of international law, beginning with international environmental law. The function of transparency in international law is similar to its role in national public law: Transparency allows for public criticism of the exercise of international governance. The principle of transparency bolsters the quality of international law as public law. It is a body of law that constitutes and channels power or authority, that seeks to serve the public interest and that is under control of the public. The transparency of international institutions and of their law-making and implementation procedures can therefore mitigate the democratic deficit of international law. (Transparency in International Law, CUP 2013 (co-ed. with Andrea Bianchi); “The Transparency Turn in International Law”, The Chinese Journal of Global Governance 1 (2015), 3-15; “Transparency Procedures”, in: Lavanya Rajamani/Jacqueline Peel (eds.), The Oxford Handbook of International Environmental Law (OUP 2nd ed. 2021), 904-919 (with Tom Sparks)).
• Territorial referendums are, when they are free, fair, peaceful and conducted under international observation, a necessary but not sufficient procedural element for the exercise of a people’s right to self-determination. Thereby, they can contribute to the legalisation of territorial changes, also in the event of a unilateral secession of a region from a state. (Das Gebietsreferendum im Völkerrecht: Seine Bedeutung im Licht der Staatenpraxis nach 1989 (Nomos 1995); “Das Völkerrecht der Gebietsreferenden: Das Beispiel der Ukraine 1991-2014”, Osteuropa (Special Issue: Zerreißprobe: Die Ukraine: Konflikt, Krise, Krieg) 64/5-6 (2014), 101-133; “The Crimean Vote of March 2014 as an Abuse of the Institution of the Territorial Referendum”, in: Christian Calliess (ed.), Liber Amicorum für Torsten Stein zum 70. Geburtstag (Nomos 2015), 278-303).

• The well-being of humans, their security and their rights constitute the foundation and the limits of state sovereignty. Humanity, not sovereignty is the Letztbegründung of international law. (“Humanity as the A and Ω of Sovereignty”, European Journal of International Law 20 (2009), 513-544).
• Individuals enjoy direct international rights and have obligations which rank “below” fundamental human rights, for example in international labour law, refugee law, international humanitarian law, and so on. (“The Direct Rights of Individuals in the International Law of Armed Conflict”, in: Jennifer Welsh/Dapo Akande/David Rodin (eds.), The Individualisation of War (OUP 2023) 58-88; Europäische Menschenrechtskonvention: Mit rechtsvergleichenden Bezügen zum deutschen Grundgesetz (2nd ed. C.H. Beck 2012 (with Tilmann Altwicker). Once we identify and acknowledge these legal positions and the international legal personhood of humans which underlies them, we can qualify the individual as an original and in normative terms “primary” subject (person) of international law whose status is not only “derived” from and subordinate to the international legal status of states. Shifting the foundation of international law from states to humans at first glance appears to be a paradigm change in international law. (Beyond Human Rights: The Legal Status of the Individual in International Law (CUP 2016). However, the protection of the interests of individuals has always been a concern of international law and prior forms of normativity since antiquity. (The Individual in International Law (co-ed. with Tom Sparks) (OUP 2024)).
• The interwar period from 1919 to 1945 was a decisive phase for the emergence of the idea and practice of treating the individual as an international legal person. Novel international regimes on refugees, stateless, minorities, and mandate populations were the fall-out of war and revolution; and diagonal arbitration was a tool to resolve both war reparation matters and satisfy the needs of the transnationalised commerce. These were accompanied by a rich doctrinal reflexion on international legal personhood, in the context of legal debates on international “democracy” and “solidarity”. It took both an internal methodological renovation away from legal positivism to a more sociological approach to international law and the external shock of the horrors of National socialism and World War II to (re-)unite the new technicalities (including procedures) with earlier and more principled considerations on the inviolability of the human person. That combination then allowed the development of a practical scheme of international human rights protection whose (symbolic) kick-off was the UDHR of 1948. (“Before Human Rights: The Formation of the International Legal Status of the Individual, 1914-45”, in: Tom Sparks/ Anne Peters (eds.), The Individual in International Law: History and Theory (OUP 2024), 119-163).
• The observation that international law has accommodated human individuals, and that individual status, rights and obligations have become denser and more important in the international legal system can be captured as the “humanisation” thesis. The humanisation thesis has faced three important but productive critiques, namely that such humanisation is neoliberal, neo-colonial, and anthropocentric. Responding to these critiques, a renewed “humanisation” can be distinguished from mere individualisation; it rather situates individuals within communities and social contexts. The humanisation of international law can and should also engage with ecocentrism and posthumanism and avoid an ecologically disastrous anthropocentrism. Thus, international law can be seen as evolving and can be made to evolve in the direction of an ecologically embedded and community-oriented humanism that is not post- or transhumanist.
  (“The Importance of Having Rights”, Zeitschrift für ausländisches öffentliches Recht und Völkerrecht 81 (2021), 7-22; “Introduction” and “Conclusion” (together with Tom Sparks), in: Tom Sparks/Anne Peters (eds.), The Individual in International Law: History and Theory (OUP 2024).
• The policy agendas of anti-corruption and human rights have been converging on the international and regional levels. This legal-political strategy of individualising the systemic problem of corruption bears risks, but the opportunities outweigh them. Doctrinally, corruption can be conceptualised as a human rights violation that triggers international state responsibility. Moreover, human rightsseem to be a proper normative framework to denounce and combat corruption. The currently one-sided integration of corruption concerns into the human rights machinery should be supplemented by a full attention to human rights in all monitoring schemes in the various anti-corruption regimes. Then, the relevant policies will likely create a positive feedback loop in which anti-corruption is instrumental to improving the human rights situation while a range of human rights will work as enablers for fighting corruption. (“Corruption as a Violation of International Human Rights”, European Journal of International Law 29 (2018), 1251-1287; “Human Rights and Corruption”, in: Mark Pieth/Tina Søreide (eds.), Elgar Concise Encyclopedia of Corruption Law (Elgar 2023), 252-256; “Human Rights and Corruption: Problems and Potential of Individualising a Systemic Problem”, International Journal of Constitutional Law 22 (2024)).

• The animal question has gone global. Human behaviour towards nonhuman animals has led to animal misery and death up to the extinction of entire animal species on a global scale. It moreover has ethical, ecological, and social implications for all humans and for the entire planet. Some of these problems, such as resource degradation and global warming, have a per se global quality and global proportions. Other issues linked to the excessive economic exploitation of animals, such as human poverty, pollution, corruption, and not the least animal suffering acquire a global dimension by the fact that animal use is embedded in the context of global capitalism, performed in transnational supply chains, and subject to a global regulatory competition. These globalised problems need globalised law and policy responses in order to be effectively regulated in the face of economic globalisation, as a contribution to global sustainable development, and as an imperative of global justice. For these reasons, global animal law has emerged as a body of law that warrants scholarly exploration, with due attention to legal and cultural pluralism.
  (“Global Animal Law: What it is and why we need it”, in: Transnational Environmental Law 5 (2016), 9-23; “Tierwohl als globales Gut: Regulierungsbedarf und -chancen”, Rechtswissenschaft (2016), 363-387; Studies in Global Animal Law (Springer 2020); Animals in International Law, Collected Courses of The Hague Academy of International Law 410 (Brill 2020), 95-544 (Livre de poche Brill 2021); “Animal Rights”, in: Christina Binder/Manfred Nowak/Jane Alice Hofbauer/Philipp Janig (eds.), Elgar Encyclopedia of Human Rights (Elgar 2022), 129-135; “Animals”, in: Sué González Hauck/Raffaela Kunz/Max Milas (eds.), Public International Law: A Multi-Perspective Approach (Routledge 2024), 305-312; Anne Peters/Kristen Stilt/Saskia Stucki (eds.), Oxford Handbook of Global Animal Law (OUP forthcoming 2025)).
• Animals are the neglected victims of armed conflict. Wildlife populations usually decline during warfare, with disastrous repercussions on the food chain, on fragile ecosystems and precarious habitats. Belligerents take advantage of the war chaos for poaching and trafficking of animal products. Lifestock, companion, and zoo animals, highly dependent on human care, are direct victims of hostilities. International humanitarian law must and can be interpreted dynamically and further developed through new legal instruments in order to improve the protection of wild animals, domestic animals, and animals employed in military operations. (Animals in the International Law of Armed Conflict (ed. with Jérôme de Hemptinne/Robert Kolb (CUP 2022) (European Society of International Law collaborative book prize 2023); “Animals in War: At the Vanishing Point of International Humanitarian Law” (together with Jérôme de Hemptinne), International Review of the Red Cross 104 (2022), 1285-1314).

• International law in the context of war and peace (ius contra bellum; ius in bello and ius post bellum) is characterised by controversies and contestations and should therefore be dealt with in a multi-perspective manner (Max Planck Trialogues on the Law of Peace and War (Series ed. with Christian Marxsen, CUP): Self-Defence against Non-State Actors ─ Vol. 1 2019 (Mary-Ellen O'Connell/Christian Tams/Dire Tladi); Law Applicable to Armed Conflict ─ Vol. 2 2020 (Ziv Bohrer/Janina Dill/Helen Duffy); Reparation for Victims of Armed Conflicts ─ Vol. 3 2020 (Cristián Correa/Shuichi Furuya/Clara Sandoval); Armed Intervention and Consent ─ Vol. 4 2023 (Dino Kritsiotis/Olivier Corten/Gregory H. Fox); The UN Security Council and the Maintenance of Peace in a Changing World ─ Vol. 5 2024 (Congyan Cai/Larissa van den Herik/Tiyanjana Maluwa)).
• German constitutional and military law (Wehrrecht) seek to balance antagonist requirements: On the one hand, Germany seeks to be a reliable partner and participant in multinational military operations. On the other hand, military operations abroad need a sufficiently strong constitutional justification. The answer is a relatively extensive parliamentary (and weaker judicial) control. The German system illustrates how minimum standards of democracy and rule of law can be upheld in the context of foreign action. (“Between Military Deployment and Democracy: Use of Force under the German Constitution”, Journal on the Use of Force and International Law 5 (2018), 246-294; “Military Operations Abroad Under the German Basic Law”, in: Curtis A. Bradley (ed.), The Oxford Handbook of Comparative Foreign Relations Law (OUP 2019), 791-809).

• The histories of international law can be written with the help of global history approaches. These approaches render us more sensitive to the problem of Eurocentrism in the evolution of international law and in its historiography. They allow us to recognise better and to appreciate more the extra-European influences. (Oxford Handbook of the History of International Law (OUP 2012) (co-ed. with Bardo Fassbender) (American Society of International Law Certificate of Merit); “The Journal of the History of International Law: A Forum for New Research”, Journal of the History of International Law (JHIL) 16 (2014), 1-8 (with Emmanuelle Tourme Jouannet); Politics and the Histories of International Law: The Quest for Knowledge and Justice (co-ed. with Raphael Schäfer) (Brill 2021)).

• International legal scholarship has an inbuilt risk of epistemic nationalism: Scholars often espouse standpoints which result from their pre-formation in their national legal system and/or which lie in the national interest of their home state. International legal scholarship should problematise and tackle this phenomenon.
• The decentralised, fragmented, politicised, and frequently inconsistent character of public international law has become especially visible in the current phase of global power shifts. These features suggest that a cultural, political, and methodological pluralism of approaches is especially adequate for international legal scholarship. Such pluralism should be used as a heuristic device in order to interrogate our observer standpoint (multiperspectivism).
• Group ascriptions such as “whiteness”, “blackness or similar are not a source (and container) of strictly segregated knowledge bubbles. However, the scholars’ life experience, their affectedness by concrete legal norms, and their general worldview (Vorverständnis) may legitimately shape the legal argument (standpoint epistemology). This does not necessarily undermine the scholarly quality of the analysis, especially as each and every participant in a debate is inevitably situated somewhere.
• The implicit privileging of the knowledge production of Western (“white”) legal scholars currently prevents a genuinely global legal multi-perspectivism. The everyday experience that the validation of scientific knowledge claims is often not based on careful examination of the arguments, but on ignorance, dogma, or other irrelevant aspects should not lead to dismissing universal quality criteria. Rather, the experience of hegemony shows how important it is to insist on arguments about international law that are comprehensible among speakers across cultures. In addition, active institutional measures must be taken to intensify a hierarchy-free transfer of ideas in all directions. The pursuit of constructive “realistic” utopias is a task of international law scholarship, and this includes keeping a critical distance from legal practice. (“Die Zukunft der Völkerrechtswissenschaft: Wider den epistemischen Nationalismus”, Zeitschrift für ausländisches öffentliches Recht und Völkerrecht 67 (2007), 721-776; “Rollen von Rechtsdenkern und Praktikern - aus völkerrechtlicher Sicht”, in: Berichte der deutschen Gesellschaft für Völkerrecht, Vol. 45 (C.F. Müller 2012), 105-173; “Realizing Utopia as a Scholarly Endeavour”, European Journal of International Law 24 (2013), 533-552; “International Legal Scholarship Under Challenge”, in: Jean d'Aspremont/Tarcisio Gazzini/André Nollkaemper/Wouter Werner (eds. ), International Law as a Profession (CUP 2017), 117-159); “Trialogical International Law”, in: Anne Peters/Christian Marxsen (eds. ), The Max Planck Trialogues on the Law of Peace and War - Introduction to the Series (CUP 2019), xi-xxv; “The Rise and Decline of the International Rule of Law and the Job of Scholars”, in: Heike Krieger/Georg Nolte/Andreas Zimmermann (eds.), The International Rule of Law: Rise or Decline? (OUP 2019), 56-65); “Semper Apertus: Wider den epistemischen Rassismus“, in: Koloniale Kontinuitäten im internationalen Recht (Berichte der Deutschen Gesellschaft für Internationales Recht Vol. 52 C.F. Müller 2024) (ed. with Eva-Maria Kieniniger and Stephan Hobe)).