Regulating Military Force Series – Hybrid Warfare and Jus ad Bellum


| Feb 20, 2024

Hybrid warfare

Editors’ note: The author delivered remarks on the subject of this post at the conference “International Law and the Regulation of Resort to Force: Exhaustion, Destruction, Rebirth?” at the Centre for International Humanitarian and Operational Law, Palacký University in Olomouc, Czech Republic.

Although not formally defined in international law, in military doctrine hybrid warfare is the synergy of effect wrought by one State upon an adversary State by combining irregular and regular military forces on the battlefield as well as in the broader battlespace. Hybrid warfare operations employ a range of non-forcible as well as forcible acts.

Popularized in military circles by Frank Hoffmann since 2007, hybrid warfare is not new as a concept. In a notable early instance, Athens stirred up a helot uprising in Sparta during the Peloponnesian War in the fifth century BCE, causing its enemy to sue for terms fearful of a threat at its own backdoor (Mansoor, Hybrid Warfare p. 3–4). But, as NATO has observed, owing to rapid technological change and global interconnectivity, propaganda, deception, sabotage, and other non-military tactics have become far more effective means to undertake and achieve deeper penetration.

Although contemporary military doctrine on hybrid warfare emanated in the United States, Russia has been its principal architect on the battlefield. Ukraine in 2014 is cited as the emblematic case study, but already in 2008 Russia demonstrated what outcomes could be achieved by its mastery in Georgia. The short-lived international armed conflict was one of the first instances in which cyberspace superiority had a substantial impact on conventional military operations. Hybrid operations engaged a cyberspace campaign in support of a conventional, kinetic ground attack, with “patriotic hackers”—seemingly operating under the direction of Russian authorities—executing the cyberattacks in support of Russian armed forces (Bryant, International Conflict and Cyber Space Superiority, p. 142-43).

Responding Forcibly to Hybrid Warfare

The exploitation of non-State armed groups (along with the cyber operations of “patriotic” individuals) by hybrid warfare does, though, cause significant problems for the application of jus ad bellum rules. In any given case, these problems are first and foremost evidentiary, but they also pertain to the interpretation and application of the law even when the evidence of foreign meddling is overwhelming. This applies both to the responsibility of the State concerned and to the right of the victim State to use force in self-defence. Has the relevant non-State armed group been “sent” by the foreign State, as in the customary law definition of aggression? What does the “substantial involvement” of that State entail with respect to the instigation of the right to use force in self-defence?

The firm views of the United States notwithstanding, it remains disputed among States and commentators whether a non-State actor can launch an armed attack against a State without being sent or instructed and equipped to do so by a State. The better view is that no instant change of customary rules governing self-defence occurred in the aftermath of the 9/11 attacks, despite the close attention paid to the preambular wording of UN Security Council Resolutions 1368 and 1373. This in turn greatly constrains the legality of a forcible response on the territory of a State where members of that non-State actor are located.

Nonetheless, on the international plane, it is still open to a victim State to pursue other political, diplomatic, and economic measures. This includes the options: to have recourse to the UN Security Council for an authorization to use force; to undertake law enforcement measures using a level of force that does not breach Article 2(4) of the UN Charter; and to apply targeted countermeasures against a responsible State that has not sent a non-State armed group to attack it. The choice of response to international terrorism is not a binary one between massive use of force against another State and/or a non-State armed group on that State’s territory on the one hand, and no possibility to use any force at all on the other.

State Responsibility for Aggression by Hybrid Warfare

Whether a supportive State bears international responsibility for the unlawful acts of a non-State armed group is at the heart of legal accountability for hybrid warfare. Absent the level of dependence required for a non-State actor to have been sent to perpetrate aggression, the “effective control” standard first set by the International Court of Justice in its judgment on the merits in the Paramilitary Activities case (para. 115), and reaffirmed two decades later in its judgment on the merits in the Bosnian Genocide case (paras. 399–400), remains the test to apply for State responsibility.

This is so, despite the efforts of Judge Antonio Cassese in the judgment on appeal in the Tadić case before International Criminal Tribunal for the former Yugoslavia to lower the threshold for attribution to one of overall control. Such control exists where a State provides financial support and/or weapons and training to a non-State armed group but where the supportive State does not give instructions for the specific operation at issue.

Thus, in the case of the attacks by Hamas against Israel on 7 October, the role of Iran, which has given Hamas weapons and funding for many years, has been closely scrutinized. Although the facts are not yet fully known, it may well be that the overall control threshold was reached in this instance, but not that of effective control.

By contrast, in theory, Russian actions in March and April 2014 in relation to the attacks on State authorities in the Donbas region of Ukraine might amount already to the crime of aggression. In August 2014, Russia covertly sent into Ukraine heavy artillery and tanks to the separatists, along with tens of thousands of existing or former members of its armed forces. This amounted not only to an international armed conflict for the purposes of the application of the law of armed conflict (given Russia’s overall control over the People’s Militia of the Luhansk People’s Republic and People’s Militia of the Donetsk People’s Republic), but also to an act of aggression for which Russia continues to bear responsibility under international law. This is the result of the effective control they then also exercised over the proxy forces.

It is clear to all that, as things stand, the regime headed by President Putin has no intention whatsoever of complying with the requirements of that legal responsibility. But there may come a time in which a new Russian government wishes to return to compliance with international law and accept the consequences of its earlier aggression in 2014 (let alone the war of aggression it launched openly with its own armed forces on 24 February 2022). Should such a scenario ever come to pass, a model for the assessment and disbursement of reparations exists that could be instituted to assess and disburse compensation as part of the duty of reparation for an internationally wrongful act.

The United Nations Compensation Commission (UNCC) was created to address Iraq’s financial liability for its unlawful invasion and occupation of Kuwait in 1990. In its Resolution 687, the UN Security Council reaffirmed that Iraq was “liable under international law for any direct loss, damage—including environmental damage and the depletion of natural resources—or injury to foreign Governments, nationals and corporations as a result of its unlawful invasion and occupation of Kuwait” (para. 15). The Commission was established in 1991 as a subsidiary organ of the Security Council under Resolution 692. While it was “neither a court nor a tribunal with an elaborate adversarial process,” in the following two decades it disbursed total compensation of approximately U.S. $48 billion to some 1.5 million successful claimants.

Concluding Thoughts

The enormity of the violation of international law that is a war of aggression does not mean that international law cannot deal with the consequences. This applies to hybrid warfare as it does to kinetic warfare between the armed forces purely of States.


Stuart Casey-Maslen is Extraordinary Professor of International Law at the Centre for Human Rights of the University of Pretoria.




Photo credit: Unsplash



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