The aftermath of an Israeli strike on a building is seen after the ceasefire between Israel and Iran, in Tehran, Iran, June 25, 2025. (OSV News/West Asia News Agency via Reuters/Majid Asgaripour)
by Gregory M. Reichberg
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There is no greater spur to wrongdoing than the fear of being wronged oneself. Here, I summarize the thought of Hugo Grotius, the great Dutch jurist, whose systematization of just war ethics remains a fundamental point of reference in our own day.
Grotius placed the idea of just cause at the heart of his doctrine. War inevitably causes immense damage, so those who embark on this course of action have a responsibility in conscience to assess whether it can be justified by moral principle. Only then will strategic advantage, likelihood of success and the balance of harm over beneficial results become morally relevant.
Absent a just cause, compliance with the other just war criteria — legitimate authority, last resort, proportionality, etc. — will fail to sway the moral balance. At best they will serve as mitigating factors for an action that is otherwise wrong.
Can there be just cause for purely preventive military action — action that aims to impede or destroy an adversary's capacity to wreck future harm? To this question, Grotius responded with a resounding no.
True, he acknowledged, defense against ongoing or imminent attack does eminently count as a just cause, provided of course the other party has attacked first, or is about to do so. If the attack is not imminent but is merely possible at some undetermined future point in time, the same justification does not hold. It patently stands against justice to bring wrath upon a person or state that might do wrong.
Retributive justice is backward looking only; it cannot, must not, be administered in advance. To think otherwise is to make fear the guiding principle in human affairs. To act on fear alone, without a grounding in truth and justice, unwinds the very fabric of societal order. International anarchy is the result. The First World War made abundantly manifest how devastating this can become.
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In writing about preventive war, Grotius had before him the example of the ancient Greek city-states, as recounted in Thucydides' famous history of the Peloponnesian War. Thucydides recounts how the Spartans decided to attack Athens before its military force grew and became unstoppable — while the "window of opportunity" was still open (as we would say today). In other words, it was fear of future harm that impelled the Spartans to initiate a war against Athens and its allies.
Thucydides suggests that this involved strategic miscalculation. While Sparta nominally won over Athens, the years of ensuing warfare seriously undermined the well-being of both city-states.
Grotius sought to draw out the moral lesson, hence his strictures against preventive war.
On June 13, Israel attacked Iran preventively. Prime Minister Benjamin Netanyahu made clear he cannot live with the possibility of a nuclear-armed Iran, claiming this represents an "existential threat" to his nation. Apparently, Israel's possession of nuclear arms (an open secret) is, in his eyes, an insufficient deterrent against a future nuclear attack by Iran.
The stated justification has built on an elaborate series of what-ifs, but all come down to a calculus of fear around Iran's growing capacity to do Israel future harm.
Nine days later, the United States joined Israel's preventive war against Iran, with attacks against Iranian nuclear sites, including the tunnel complex under Fordo. The exact level of destruction at the time of writing remains unclear. President Donald Trump claims the sites have been "obliterated," while other estimates range from severe destruction (requiring several years to rebuild), to more limited damage that could be undone over a span of several months.
People look at the apparent remains of a ballistic missile following a missile attack by Iran on Israel, in northern Israel, June 24, 2025. (OSV News/Reuters/Avi Ohayon)
It remains unknown whether, before the attacks took place, Iran managed to transport into secret locations its considerable stock of enriched uranium, keeping it safe from the bombing that subsequently occurred.
On June 24, the three countries agreed to a ceasefire. The world awaits with bated breath to see how Iran eventually responds. Will it renounce its nuclear program and embrace a grand bargain that could afford it with economic prosperity? Or, more likely, will it safeguard its cache of enriched uranium and persist in its longstanding stance of resistance? Will the experience of being bombed lead Iran to abandon its official opposition to nuclear weapons — as attested by Supreme Leader Ayatollah Ali Khamenei's fatwa prohibiting his country's development of a nuclear capability — and instead race to make a bomb as insurance against future attack?
Whatever the outcome, these words of Grotius speak for themselves:
Quite untenable is the position, which has been maintained by some, that according to the law of nations it is right to take up arms in order to weaken a growing power which may do harm should it become too great. ... That this consideration does enter into deliberations regarding war, I admit, but only on ground of utility, not of justice. ... That the possibility of being attacked confers the right to attack is abhorrent to every principle of equity. Human life exists under such conditions that complete security is never guaranteed to us.
Grotius' message is clear: Fear alone offers no justification. It promises a world where the security dilemma becomes the norm; strike first before I am struck. Do we want to live in such a world?
Author's note: This article is rewritten for National Catholic Reporter from original work appearing at Peace Research Institute Oslo. It draws on Gregory M. Reichberg's book Thomas on War and Peace, published by Cambridge University Press in 2017.
