More than two months into the war in Iran, navigation through the Strait of Hormuz – the key waterway through which more than a third of the international trade in oil and gas passes – remains perilous and uncertain. Underscoring the uncertainty, on May 3, 2026, the Trump administration launched Project Freedom to help stranded ships through the strait. Yet the next day, at least two ships came under fire from Iran.

Iran began blocking the strait to navigation on Feb. 28, after the United States and Israel launched a military campaign against the country. By mid-March, Tehran was demanding tolls of up to US$2 million per vessel. In response, the U.S. imposed what President Donald Trump declared to be a “complete” maritime blockade on Iran and subsequently threatened punishing economic sanctions on any entity that pays Iran’s tolls.

Following Iran’s lead, other nations are now contemplating using their own leverage over crucial choke points closer to their shores. Indonesia floated a proposal to charge tolls on vessels transiting the Strait of Malacca, before walking it back. China has also issued warnings against foreign military vessels transiting the Taiwan Strait.

These events have prompted commentators to warn of the end of a golden era of navigational freedom that the U.S. has underwritten for more than a century. But as an expert on international law, I know that attempts by nations to weaponize their leverage over crucial geographic choke points at sea and on land are nothing new. In fact, they go back at least six centuries.

The Danish roots of sea tolls

From the early 15th century until 1857, Denmark required ships passing through the narrow straits connecting the North Sea to the Baltic Sea to stop at the port city of Helsingør — or Elsinore, as Shakespeare styled it in “Hamlet” — and pay a toll before proceeding.

At their peak, these Sound Dues generated nearly 10% of Danish national revenues. The Sound Dues rankled the maritime powers of the day, but Denmark could easily enforce them thanks to the narrowness of the Øresund Strait, which is less than 3 miles wide at Helsingør.

Ultimately, they were ended not through war but through diplomacy, led in large part by a rising maritime power with a strong interest in open sea-lanes: the United States.

Seeking to increase its trade with Prussia, in 1843 the administration of President John Tyler advised Denmark of the United States’ refusal to pay the Sound Dues because they lacked any basis in international law. Rumors swirled that the U.S. was willing to back up its refusal to pay with force.

After years of uncertainty, the fate of the Sound Dues was resolved by the Copenhagen Convention of 1857. Denmark agreed to abolish the tolls forever in exchange for a one-time, lump-sum payment from the major trading nations. The principle of free navigation of the world’s oceans has largely prevailed since then, in part as a result of subsequent U.S. efforts to exercise these freedoms against those who would restrict them.

How the law developed

The Danish settlement reflected a broader body of law – the law of transit – that had been evolving alongside an international system of sovereign states for centuries.

Its core principle is that when convenience dictates or necessity requires, a country must allow the people, goods and vessels of other nations to pass through its territory for a journey that begins and ends elsewhere. The principle has deep roots in American and international legal history: Thomas Jefferson invoked it when negotiating with Spain, which then controlled Louisiana, to secure the United States’ right to navigate the Mississippi River.

Free transit guarantees have been a feature of every major international order since the Congress of Vienna ended the Napoleonic wars in 1815. Yet in each case, those guarantees have come under pressure as the order that produced them weakened.

Before World War I, restrictions on transit rights multiplied across Europe. The League of Nations, a precursor to today’s United Nations, made strengthening transit rights its first priority in the 1920s. But these arrangements fell apart as fascism rose across Europe and Asia and regimes from Nazi Germany to Imperial Japan denounced their international legal obligations.

The post-World War II order reaffirmed transit rights – through the law of the sea, trade agreements and the laws governing civil aviation – and for decades they held.

The International Court of Justice clarified the governing legal principle for international straits in its very first case, decided in 1949: Any body of water useful to international navigation between two open seas is open to the vessels of all nations.

The U.N. Convention on the Law of the Sea, concluded in 1982, reaffirmed this rule in holding that countries may not charge tolls on vessels passing through straits within their waters. Although neither Iran nor the U.S. has ratified the convention, the U.S. accepts its provisions on navigational freedom as binding on all countries.

Iran’s levying of tolls in the Strait of Hormuz violates the core legal principle that nations may not exploit advantages of geography to bilk foreigners who need to traverse their land or maritime territory. Yet the American and Israeli military campaign that provoked Iran’s response likewise violates the U.N. Charter’s rules on the use of force.

Such issues are not just limited to the Strait of Hormuz. Indeed, trade law, security commitments and the norms against the unilateral redrawing of borders are all under strain.

Seen in this larger context, China’s warnings against military passage through the Taiwan Strait and Indonesia’s trial balloon over the Malacca Strait are not isolated provocations. They are symptoms of the same underlying condition: an international order losing the shared commitment that has often made its rules enforceable.

In January 2026, Trump told The New York Times that he did not need international law and that his own moral judgment was the only constraint on American foreign policy. Around the same time, Canadian Prime Minister Mark Carney warned that the American-led international order was “fading.”

The Strait of Hormuz is where those trend lines are now colliding – to the detriment of billions of people around the world, and to the idea of an international order based on law rather than the naked exercise of power.

Vivek Krishnamurthy does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.