By Brahma Chellaney, The Japan Times

A naval blockade is not merely a coercive tactic to prevent vessels from entering or leaving a country’s ports. It is an act of war.

History points out that this distinction matters. Yet it is precisely this line that the U.S. under President Donald Trump chose to blur through its naval blockades of Cuba and Iran.

By straining the bounds of international law, those actions revived a historically dangerous logic: that economic strangulation can be used as a substitute for open conflict without triggering similar consequences.

International law is unambiguous on this point. The London Declaration on the Laws of Naval War (1909) and the San Remo Manual on International Law Applicable to Armed Conflicts at Sea (1994) set out clear criteria for when a blockade may be considered lawful. These include requirements of prior declaration, proportionality, impartiality and the protection of civilians. The U.S. blockades of Cuba and Iran scarcely met and arguably violated these standards.

The humanitarian consequences are already starkly visible in Cuba. The blockade, in place since Jan. 29, has contributed to a worsening humanitarian crisis marked by grid collapse, water scarcity, failing hospitals and shortages of food and basic goods for Cuba’s 11 million people, with hunger spreading. Trump has been explicit about the endgame: “taking Cuba.”

Under Protocol I of the Geneva Conventions (1949), starving civilians as a method of warfare, or inflicting disproportionate suffering on a population, is explicitly prohibited and constitutes a war crime. So, too, is collective punishment: penalizing an entire population for the actions of its government or military.

The blockade of Iranian ports carried serious international implications, as the world economy was already reeling from the largest energy-supply disruption in history, triggered by the 40‑day U.S.-Israeli war on Iran that began on Feb. 28. Trump’s Iran blockade was not merely a violation of the April 8 ceasefire understanding; it amounted to the initiation of new hostilities by other means.

Under Article 51 of the United Nations Charter, nations have a right to self-defense only in response to an “armed attack.” A naval blockade, by contrast, is generally regarded in international law as an initial act of aggression.

History offers sobering lessons about where such actions can lead.

In 1962, the U.S. under President John F. Kennedy imposed what it termed a “quarantine” on Cuba — a linguistic maneuver that did little to disguise the reality of a blockade. The result was the Cuban Missile Crisis, which brought the world to the brink of a calamitous U.S.-Soviet nuclear war.

The crisis was ultimately defused not because the blockade was stabilizing, but because both sides stepped back from the abyss.

Two decades earlier, the U.S. had pursued a different form of economic strangulation: the 1941 oil embargo on Japan. While not a naval blockade in the strict sense, it had a comparable effect — cutting off vital supplies to a major power. Tokyo interpreted the embargo not as a reversible economic measure but as an existential threat.

Widely seen as the point of no return, it led to Japan’s attack on Pearl Harbor — which, in turn, triggered a protracted Pacific War culminating in the U.S. nuclear destruction of Hiroshima and Nagasaki.

The pattern is difficult to ignore. Measures intended to coerce through deprivation are often perceived by their targets as acts of war, demanding a forceful response. In such circumstances, legal distinctions that may appear clear in theory become secondary to the imperatives of survival and deterrence.

This is the central danger of blockade as a strategy. It rests on a fragile assumption: that one side can impose escalating pressure while retaining control over the threshold of war.

Yet history suggests otherwise. Once a capable, resilient nation is subjected to what it perceives as strangulation, the incentives shift toward escalation, often in unpredictable and asymmetric ways.

For the U.S., the risks from the latest blockades extended beyond reputational costs to the danger of strategic miscalculation.

More ominously, the normalization of blockade as a tool of statecraft risks further eroding the already-fragile constraints on the use of force in international relations. If powerful states can act this way without clear legal justification, the precedent will not remain confined. Others will follow and the distinction between war and peace will become increasingly difficult to sustain.

There is a reason why international law has sought to regulate — and in many respects restrict — the use of blockades. It reflects a hard-earned recognition that strategies of strangulation carry within them the seeds of major escalation. They do not freeze conflicts; they intensify them and widen their human cost.

The lesson of both the Pacific War and the Cuban Missile Crisis was stark: When a nation is being strangled, restraint becomes politically and strategically untenable.

In the nuclear age, strangulation is not just coercion; it is a gamble with uncontrollable escalation.

Brahma Chellaney, a longtime contributor to The Japan Times, is the author of nine books, including “Water: Asia’s New Battleground.”