When news coverage describes an airstrike as “disproportionate,” what does that actually mean under international law? The word is often used loosely as a synonym for “excessive” or “unjustified.” But the legal principle of proportionality in International Humanitarian Law (IHL) has a precise and demanding definition. Understanding this distinction matters enormously for accountability efforts in conflicts like Russia’s ongoing war against Ukraine. Applying this rule requires considering difficult concepts like “dual-use” infrastructure, predicting long-term effects, and proving what a military commander knew before a strike.

What Is the Proportionality Rule?

The principle of proportionality is codified under IHL as a specific prohibition against indiscriminate attacks, detailed in Article 51(5)(b) of Additional Protocol I to the Geneva Conventions. It prohibits any attacks when the expected civilian harm—measured in civilian casualties, injuries and damage to civilian objects—is excessive in relation to the anticipated concrete and direct military advantage of the attack.

Two things about this standard are worth emphasizing. First, it does not prohibit civilian harm altogether; IHL accepts that warfare causes civilian casualties. What it prohibits is excessive harm—harm that is clearly disproportionate to a genuine, identifiable military gain. Second, this assessment is made from the perspective of a reasonable military commander at the time of the attack, relying entirely on information available before the strike regarding its anticipated effects—not the strike’s actual outcome.

This makes proportionality one of IHL’s most complex and contested principles. It requires a subjective comparison of fundamentally different values: military advantage on one side, and protected human life and civilian infrastructure on the other.

Applying Proportionality: Russia’s Attacks on Ukraine’s Energy Infrastructure

Russia’s sustained campaign against Ukraine’s civilian power grid offers one of the most documented and widely analyzed applications—and alleged violations—of the proportionality principle.

The campaign’s scale is staggering. By May 2025, approximately 90 percent of Ukraine’s thermal power generation had been destroyed, and the country’s energy infrastructure was operating at roughly one third of its pre-invasion capacity. The winter of 2025–2026 brought the most severe documented impacts yet. In January 2026 alone—one of the harshest winters Ukraine has experienced since the invasion began—Russian forces conducted near-daily strikes on energy infrastructure, including five large-scale attacks in which multiple regions were simultaneously targeted, damaging or destroying key components of the energy system in at least 17 regions and Kyiv city. Millions of people were left with electricity for only a few hours a day throughout the month, amid temperatures that fell well below freezing. Emergency power cuts affected 80 percent of the country. By February 2026, President Zelenskyy stated that every power plant in Ukraine had been damaged by Russian attacks; at one point, roughly half of Kyiv’s 12,000 apartment buildings lost heating entirely.

Under IHL, civilian infrastructure such as power plants and heating systems is not automatically protected from attack. If these facilities power military installations or command centers, they are classified as dual-use objects. This means that while they make a direct and effective contribution to military operations and may qualify as legitimate military targets, their attack is still strictly bound by the laws of war. Critically, IHL establishes that attacks on energy grids must be avoided if they will disproportionately affect a large civilian population, as electricity is indispensable to civilian survival. The deliberate destruction of infrastructure that serves an overwhelmingly civilian function—particularly when timed to maximize suffering during the coldest months—raises serious questions about whether any anticipated military advantage could be proportionate to that harm. Indeed, the UN Office of the High Commissioner for Human Rights said this about the attacks: the destruction of critical energy infrastructure, including heating facilities during winter, breaches IHL principles designed to protect civilians.

Why Proportionality Is Hard to Prove

Despite its importance, proportionality is one of the most difficult IHL rules to enforce in practice. Because the test is applied from the perspective of the attacker at the moment of the decision—based on what the commander knew or should have known—proving a violation requires evidence of the commander’s intent, knowledge and decision-making process. That information is rarely documented and even more rarely accessible to investigators and prosecutors.

Furthermore, when targeting power grids, is must be proven that the commander anticipated not just the immediate damage, but the long-term, indirect effects—such as hospital blackouts or water shortages—and assessed them to be excessive relative to the military gain. This is why evidence collection—including open-source intelligence and satellite imagery—is so critical to proving a violation of the proportionality rule. Documenting the scale, timing and effects of strikes builds the evidentiary record needed to demonstrate that an attack could not plausibly have been considered proportionate by any reasonable military commander. This forensic approach is exactly what allowed the issuance of international arrest warrants against senior Russian officials specifically for causing excessive incidental harm during grid strikes.

The Bigger Picture

Proportionality is not a loophole that legalizes civilian suffering. It is a strict international legal obligation that requires military planners to take feasible precautions, weigh human costs before any attack—and to call off strikes when the harm to civilians would be excessive. When that obligation is ignored systematically and at scale, it becomes an international crime.