Authored By: PRISCILLE MABWA MBONGO
Graduate
Abstract
Due diligence has become a defining principle of contemporary international law, yet its legal significance remains contested. Traditionally understood as an obligation of conduct rather than one of result, the doctrine has increasingly been shaped by international jurisprudence, particularly through the evolving case law of the International Court of Justice. This article examines whether this judicial development has transformed due diligence into a meaningful basis for assessing state responsibility while exposing the continuing uncertainty surrounding its scope and legal consequences. It argues that due diligence has evolved beyond a purely procedural standard, emerging as an essential measure of responsible state conduct and international accountability.
Keywords: due diligence; international law; state responsibility; International Court of Justice.
Introduction
Due diligence has emerged as one of the most influential yet conceptually unsettled principles in contemporary international law. Initially developed as a standard requiring States to exercise reasonable care to prevent harm rather than guarantee specific outcomes, it has gradually expanded across diverse legal regimes, including environmental protection, human rights, cyberspace, and business and human rights.[1] Despite its widespread acceptance, the precise legal character of due diligence remains contested. While it is traditionally classified as an obligation of conduct, recent international jurisprudence increasingly associates inadequate due diligence with findings of international responsibility, raising important questions about whether the doctrine continues to function merely as a preventive standard or has acquired a more substantive accountability dimension.[2]
Although the growing scholarship has examined the content and application of due diligence across specialised fields, comparatively less attention has been paid to how judicial interpretation has reshaped its legal significance within the broader law of State responsibility.[3] This article addresses that gap by critically examining the evolution of due diligence through the jurisprudence of the International Court of Justice, with particular emphasis on Pulp Mills on the River Uruguay and Certain Activities Carried Out by Nicaragua in the Border Area. It argues that while due diligence remains an obligation of conduct, its judicial interpretation increasingly positions it as an essential benchmark for evaluating responsible State behaviour and attributing international responsibility for preventable harm.
II. Due Diligence as an Obligation of Conduct
Due diligence is not a single legal rule but a standard of behaviour requiring States to take all reasonable and appropriate measures to prevent significant harm within their jurisdiction or control. Rather than prescribing predetermined outcomes, it evaluates the adequacy of a State’s conduct in light of the circumstances, including the foreseeability of harm, the degree of risk involved, and the capacity of the State to respond effectively.[4] Consequently, international responsibility does not arise merely because harm occurs, but because a State has failed to exercise the level of care reasonably expected under international law.[5]
This understanding has traditionally characterised due diligence as an obligation of conduct rather than an obligation of result. States are therefore expected to employ reasonable efforts, establish appropriate regulatory frameworks, and maintain effective oversight, without guaranteeing that harmful consequences will never materialise.[6] The flexible nature of the standard has enabled its application across diverse areas of international law, including environmental protection, human rights, cybersecurity, and business and human rights. Yet this flexibility also exposes its principal weakness. Because the content of due diligence varies according to context, the threshold of compliance often remains uncertain, creating inconsistency in judicial interpretation and limiting predictability in the assessment of State responsibility.[7]
The traditional characterisation of due diligence as a procedural and preventive obligation therefore captures only part of its contemporary legal significance. As international adjudicatory bodies have increasingly relied on due diligence to evaluate whether States have acted responsibly in preventing foreseeable harm, the distinction between an obligation of conduct and the attribution of responsibility has become progressively less clear.[8] This evolution is particularly evident in the jurisprudence of the International Court of Justice.
III. Judicial Evolution toward Accountability
The jurisprudence of the International Court of Justice (ICJ) has significantly shaped the contemporary understanding of due diligence by clarifying its practical operation in the assessment of State conduct. While the Court has consistently maintained that due diligence is an obligation of conduct rather than one of result, its reasoning increasingly demonstrates that the adequacy of preventive measures has become central to determining whether a State has fulfilled its international obligations. This jurisprudential development has narrowed the conceptual divide between preventive conduct and legal accountability.
The Court’s judgment in Pulp Mills on the River Uruguay marked a decisive step in this evolution. Although the dispute centred on the interpretation of the 1975 Statute of the River Uruguay, the Court recognised that States are required to exercise due diligence in preventing significant transboundary environmental harm.[9] Importantly, it identified the environmental impact assessment as an essential procedural component of that obligation where there is a risk of significant adverse effects.[10]Rather than treating due diligence as an abstract principle, the Court linked compliance to concrete preventive measures capable of demonstrating responsible State conduct. Consequently, judicial scrutiny shifted from whether environmental harm ultimately occurred to whether the State had acted reasonably before the harm materialised.
The Court reinforced this approach in Certain Activities Carried Out by Nicaragua in the Border Area, where it reaffirmed that environmental impact assessments and other preventive measures form part of the broader obligation of due diligence whenever activities pose a risk of significant transboundary damage.[11] Although responsibility continued to depend upon a failure to exercise reasonable care rather than the occurrence of harm itself, the Court’s reasoning confirmed that omissions in preventive governance may constitute internationally wrongful conduct. Due diligence therefore emerged not merely as a procedural expectation but as the legal standard against which State behaviour is judicially evaluated.
This jurisprudential trajectory has broader implications for the law of State responsibility. Contemporary scholarship increasingly recognises that due diligence functions as the normative bridge between primary obligations of prevention and the secondary rules governing responsibility for internationally wrongful acts.[12] The significance of recent climate litigation further illustrates this development, where courts and tribunals have relied on due diligence to assess whether States have adopted sufficiently robust measures to address foreseeable environmental risks.[13] Accordingly, although due diligence remains doctrinally classified as an obligation of conduct, judicial practice demonstrates that its fulfilment has become indispensable to the determination of international responsibility. The evolution lies not in transforming due diligence into an obligation of result, but in elevating the quality of preventive conduct to a decisive criterion of legal accountability.
IV. Why the Traditional Understanding Is No Longer Sufficient
The traditional characterisation of due diligence as an obligation of conduct continues to provide an important doctrinal foundation, yet it no longer fully captures the way the concept operates in contemporary international law. Although courts consistently affirm that States are not required to guarantee particular outcomes, judicial practice increasingly evaluates compliance by scrutinising the effectiveness, timeliness, and adequacy of preventive measures rather than merely their formal adoption.[14] Consequently, the legal inquiry has shifted from whether a State simply acted to whether it acted sufficiently in light of foreseeable risks.
This evolution exposes a persistent ambiguity within the doctrine. The flexible nature of due diligence enables its adaptation across diverse legal regimes, but it also leaves the standard of reasonable care inherently indeterminate.[15] States therefore face uncertainty regarding the precise level of preventive action required to satisfy their international obligations, while courts retain considerable discretion in assessing compliance. Such uncertainty risks inconsistent application and may weaken the doctrine’s capacity to promote legal predictability.[16]
Nevertheless, this indeterminacy should not be understood as evidence of doctrinal weakness alone. Rather, it reflects the contextual nature of due diligence, which must remain responsive to evolving scientific knowledge, technological developments, and transboundary risks. The challenge for international law is therefore not to abandon the traditional conception of due diligence, but to clarify its normative content through more consistent judicial reasoning. In this respect, accountability increasingly depends not on the occurrence of harm, but on whether States can demonstrate that they exercised genuinely effective and proportionate preventive diligence.[17]
V. Conclusion
This article has argued that the traditional characterisation of due diligence as an obligation of conduct no longer fully reflects its contemporary operation in international law. Although States remain under no duty to guarantee particular outcomes, the jurisprudence of the International Court of Justice demonstrates that the adequacy of preventive conduct has become the principal criterion for assessing compliance with international obligations and, where appropriate, the attribution of international responsibility.[18] By examining this jurisprudential evolution, the article has addressed the gap between the orthodox doctrinal understanding of due diligence and its increasingly substantive role in judicial reasoning. The continuing indeterminacy of the standard nevertheless underscores the need for greater normative clarity. Future legal development should therefore focus on articulating more consistent indicators of reasonable preventive conduct while preserving the contextual flexibility that makes due diligence responsive to evolving transboundary risks. Such refinement would strengthen legal certainty without diminishing the preventive function that now lies at the heart of responsible State conduct.[19]
Bibliography
Cases
Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v Nicaragua) and Construction of a Road in Costa Rica along the San Juan River (Nicaragua v Costa Rica) (Judgment) [2015] ICJ Rep 665.
Pulp Mills on the River Uruguay (Argentina v Uruguay) (Judgment) [2010] ICJ Rep 14.
Books
Ollino A, Due Diligence Obligations in International Law (Cambridge University Press 2022).
Journal Articles
Coco A and De Souza Dias T, ‘Cyber Due Diligence: A Patchwork of Protective Obligations in International Law’ (2021) 32(3) European Journal of International Law 771–806.
Latif SM and Khan A, ‘Focus on the Responsibility of States under International Law for Environmental Damages that Cross Borders’ (2025) 8(2) Review of Education, Administration and Law 1–14.
Liang S, ‘Reflections on the Interpretation and Application of the Due Diligence Obligation in International Climate Litigation: A Comparative Study of Daniel Billy et al. v Australia and the COSIS Advisory Opinion’ (2024) Frontiers in Marine Science.
Malaihollo M, ‘Due Diligence in International Environmental Law and International Human Rights Law: A Comparative Legal Study of the Nationally Determined Contributions under the Paris Agreement and Positive Obligations under the European Convention on Human Rights’ (2021) 68 Netherlands International Law Review 121–155.
McCorquodale R and Bonnitcha J, ‘The Concept of “Due Diligence” in the UN Guiding Principles on Business and Human Rights’ (2017) 28(3) European Journal of International Law 899–919.
Peters A, Krieger H and Kreuzer L, ‘Due Diligence: The Risky Risk Management Tool in International Law’ (2020) 9(2) Cambridge International Law Journal 167–182.
Encyclopedia
Flemme M, ‘Due Diligence in International Law’ in Max Planck Encyclopedia of Public International Law (Oxford University Press 2023).
[1] Anne Peters, Heike Krieger and Leonhard Kreuzer, ‘Due Diligence: The Risky Risk Management Tool in International Law’ (2020) 9 (2) Cambridge International Law Journal 167; Andrea Coco and Talita de Souza Dias, ‘Cyber Due Diligence: A Patchwork of Protective Obligations in International Law’ (2021) 32(3) European Journal of International Law 771.
[2]Mieke Malaihollo, ‘Due Diligence in International Environmental Law and International Human Rights Law: A Comparative Legal Study of the Nationally Determined Contributions under the Paris Agreement and Positive Obligations under the European Convention on Human Rights’ (2021) 68 Netherlands International Law Review 121.
[3] Robert McCorquodale and Jonathan Bonnitcha, ‘The Concept of “Due Diligence” in the UN Guiding Principles on Business and Human Rights’ (2017) 28(3) European Journal of International Law 899; Mattias Flemme, ‘Due Diligence in International Law’ in Max Planck Encyclopedia of Public International Law (Oxford University Press 2023); Shiqi Liang, ‘Reflections on the Interpretation and Application of the Due Diligence Obligation in International Climate Litigation: A Comparative Study of Daniel Billy et al. v Australia and the COSIS Advisory Opinion’ (2024) Frontiers in Marine Science.
[4] Anne Peters, Heike Krieger and Leonhard Kreuzer, ‘Due Diligence: The Risky Risk Management Tool in International Law’ (2020) 9(2) Cambridge International Law Journal 167, 170–174; Attila Ollino, Due Diligence Obligations in International Law (Cambridge University Press 2022) 18–31.
[5] Andrea Coco and Talita de Souza Dias, ‘Cyber Due Diligence: A Patchwork of Protective Obligations in International Law’ (2021) 32(3) European Journal of International Law 771, 776–781.
[6] Mieke Malaihollo, ‘Due Diligence in International Environmental Law and International Human Rights Law: A Comparative Legal Study of the Nationally Determined Contributions under the Paris Agreement and Positive Obligations under the European Convention on Human Rights’ (2021) 68 Netherlands International Law Review 121, 126–133; Robert McCorquodale and Jonathan Bonnitcha, ‘The Concept of “Due Diligence” in the UN Guiding Principles on Business and Human Rights’ (2017) 28(3) European Journal of International Law 899, 904–909.
[7] Mattias Flemme, ‘Due Diligence in International Law’ in Max Planck Encyclopedia of Public International Law (Oxford University Press 2023); Anne Peters, Heike Krieger and Leonhard Kreuzer (n 4) 176–179.
[8] Shiqi Liang, ‘Reflections on the Interpretation and Application of the Due Diligence Obligation in International Climate Litigation: A Comparative Study of Daniel Billy et al. v Australia and the COSIS Advisory Opinion’ (2024) Frontiers in Marine Science; Attila Ollino (n 4) 245–252.
[9] Pulp Mills on the River Uruguay (Argentina v Uruguay) (Judgment) [2010] ICJ Rep 14, paras 101–102.
[10] ibid paras 197–205; Mieke Malaihollo, ‘Due Diligence in International Environmental Law and International Human Rights Law: A Comparative Legal Study of the Nationally Determined Contributions under the Paris Agreement and Positive Obligations under the European Convention on Human Rights’ (2021) 68 Netherlands International Law Review 121, 128–133; Attila Ollino, Due Diligence Obligations in International Law (Cambridge University Press 2022) 136–145
[11] Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v Nicaragua) and Construction of a Road in Costa Rica along the San Juan River (Nicaragua v Costa Rica) (Judgment) [2015] ICJ Rep 665, paras 104–109.
[12] Anne Peters, Heike Krieger and Leonhard Kreuzer, ‘Due Diligence: The Risky Risk Management Tool in International Law’ (2020) 9(2) Cambridge International Law Journal 167, 181–187; Andrea Coco and Talita de Souza Dias, ‘Cyber Due Diligence: A Patchwork of Protective Obligations in International Law’ (2021) 32(3) European Journal of International Law 771, 780–785.
[13] Shiqi Liang, ‘Reflections on the Interpretation and Application of the Due Diligence Obligation in International Climate Litigation: A Comparative Study of Daniel Billy et al. v Australia and the COSIS Advisory Opinion’ (2024) Frontiers in Marine Science; Syed Muhammad Latif and Anam Khan, ‘Focus on the Responsibility of States under International Law for Environmental Damages that Cross Borders’ (2025) 8(2) Review of Education, Administration and Law 1–9.
[14] Attila Ollino, Due Diligence Obligations in International Law (Cambridge University Press 2022) 245–252; Anne Peters, Heike Krieger and Leonhard Kreuzer, ‘Due Diligence: The Risky Risk Management Tool in International Law’ (2020) 9(2) Cambridge International Law Journal 167, 181–187.
[15] Mattias Flemme, ‘Due Diligence in International Law’ in Max Planck Encyclopedia of Public International Law (Oxford University Press 2023); Andrea Coco and Talita de Souza Dias, ‘Cyber Due Diligence: A Patchwork of Protective Obligations in International Law’ (2021) 32(3) European Journal of International Law 771, 784–789.
[16] Mieke Malaihollo, ‘Due Diligence in International Environmental Law and International Human Rights Law: A Comparative Legal Study of the Nationally Determined Contributions under the Paris Agreement and Positive Obligations under the European Convention on Human Rights’ (2021) 68 Netherlands International Law Review 121, 147–152; Syed Muhammad Latif and Anam Khan, ‘Focus on the Responsibility of States under International Law for Environmental Damages that Cross Borders’ (2025) 8(2) Review of Education, Administration and Law.
[17] Shiqi Liang, ‘Reflections on the Interpretation and Application of the Due Diligence Obligation in International Climate Litigation: A Comparative Study of Daniel Billy et al. v Australia and the COSIS Advisory Opinion’ (2024) Frontiers in Marine Science; Robert McCorquodale and Jonathan Bonnitcha, ‘The Concept of “Due Diligence” in the UN Guiding Principles on Business and Human Rights’ (2017) 28(3) European Journal of International Law 899, 915–919.
[18] Pulp Mills on the River Uruguay (Argentina v Uruguay) (Judgment) [2010] ICJ Rep 14; Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v Nicaragua) and Construction of a Road in Costa Rica along the San Juan River (Nicaragua v Costa Rica) (Judgment) [2015] ICJ Rep 665; Attila Ollino, Due Diligence Obligations in International Law (Cambridge University Press 2022) 245–252.
[19] Anne Peters, Heike Krieger and Leonhard Kreuzer, ‘Due Diligence: The Risky Risk Management Tool in International Law’ (2020) 9(2) Cambridge International Law Journal 167, 186–188; Andrea Coco and Talita de Souza Dias, ‘Cyber Due Diligence: A Patchwork of Protective Obligations in International Law’ (2021) 32(3) European Journal of International Law 771, 789–792; Mieke Malaihollo, ‘Due Diligence in International Environmental Law and International Human Rights Law: A Comparative Legal Study of the Nationally Determined Contributions under the Paris Agreement and Positive Obligations under the European Convention on Human Rights’ (2021) 68 Netherlands International Law Review 121, 151–155.
