When serving as President Jefferson’s Secretary of State, James Madison published a book on the law of the sea. Examination of the British Doctrine argues that the Royal Navy’s interference with American shipping on the high seas was in violation of the law of nations. It starts with the doctrines of the 17th-century Dutch jurist Hugo Grotius. It then proceeds to canvas the views of the Swiss diplomat Emer de Vattel, the German jurist Samuel Pufendorf, and other eminent authorities, before reviewing the decisions of British prize courts. The book is longer than all of Madison’s contributions to the Federalist Papers combined. It was less persuasive than that earlier work, however, at least with its most relevant audience. As a congressional critic said at the time, it was “a shilling pamphlet against 800 British ships of war.”

David Bosco’s Poseidon Project also starts with Grotius and the ongoing American interest in “freedom of the seas.” It is hardly a criticism to say Bosco, professor of international studies at Indiana University, does not seem as learned or acute as James Madison. His book is disappointing in other ways, however, that were not as unavoidable. The book does have the merit of taking the story up to the 21st century. Bosco shows how issues that Madison confronted were addressed by later generations. And he shows how central these issues have been both to American geo-strategy and the outlook of many nations on international law. The Poseidon Project covers a lot of ground in a brisk, engaging narrative, making it much more appealing than most academic offerings.

Before getting to the disappointments, it’s worth illustrating the charms of this account. By focusing on the law of the sea, Bosco makes particular issues, often now remembered as peripheral concerns, regain the importance—or at least the rhetorical urgency—they had in earlier times. We recall, for example, Woodrow Wilson’s claim to be fighting in Europe to “make the world safe for democracy.” But as Bosco notes, when President Wilson asked Congress to declare war on Germany in April of 1917, what he emphasized was the ”sacred” claim to travel the high seas without threat, as recognized by international law. He then proceeded to depict the U.S. resort to war as a defense of international law:

International law had its origin in the attempt to set up some law which would be respected upon the seas, where no nation had right of domination and where lay the free highways of the world.

Germany’s U-boat attacks on neutral shipping were therefore, Wilson proclaimed, “warfare against mankind.”

Wilson found it easy to make anything he favored into a “sacred” principle. But as Bosco reminds us, President Franklin Roosevelt adopted similar rhetoric in the spring of 1941, explaining why the U.S. Navy was being deployed to protect merchant ships bound for Britain.

All freedom—meaning freedom to live, and not freedom to conquer and subjugate other peoples—depends on freedom of the seas. All of American history—North, Central, and South American history—has been inevitably tied up with those words, “freedom of the seas.”

One complication for American statesmen was that the rules regarding lawful conduct on the high seas were not set out in any overarching treaty. As late as the mid-20th century, most of the “law of the sea” was customary practice, documented, clarified (and sometimes enhanced) by decisions of national prize courts, purporting to determine the legality of seizures on the seas in wartime. Well into the 19th century, international law, here as elsewhere, was still seen (as James Kent’s Commentaries put it) as “deriving much of its force and dignity from the . . . law of nature.”

By the late 19th century, however, international lawyers had become doubtful of appeals to natural law, even as a supplement or orienting guide to the law: only a positivist reading of precedents and formal agreements seemed “scientific” and “modern.” Bosco illustrates the new attitude in a reported exchange in the proceedings of an arbitration panel in 1893. American lawyers urged the right of the United States to restrict brutal bludgeoning of seals off the coast of Alaska (but beyond U.S. territorial waters) as sanctioned by “the Great Book of Nature.” One of the arbitrators immediately objected: “What are your authorities? My Lord, The Book of Nature—what page and what edition?”

But the world wars saw the U.S. Navy engage in practices that were not just straining traditional views about the law of the sea at the margins. Already in the First World War, Britain and France had extended their blockade of German ports to cover nearby neutral ports, lest goods be carried from there into Germany by land. By the time the U.S. Navy joined their efforts, the Allies had claimed the right to search all neutral ships on the high seas to ensure they were not carrying forbidden cargo to forbidden destinations—prompting remaining neutral states to submit to inspection and certification by Allied officials of merchant cargoes leaving their own (neutral) ports. In effect, access to the Atlantic was licensed by the Allied powers. In the Second World War, the U.S. Navy essentially adopted German U-boat tactics in the Pacific, attacking any merchant vessel with Japanese markings or heading to Japanese ports.

Bosco’s account brushes lightly over the extent to which this departed from prior understandings. He certainly recognizes that the world wars proved a strain on older practices. But he is keen to keep his account moving forward to notice later challenges, which then come to appear in Bosco’s account as almost analogous. After recounting a series of earlier disputes about transit rights and fishing rights (notably the “Cod War” between Britain and Iceland in the 1970s), he focuses on the negotiation of the 1982 UN Convention on the Law of the Sea (UNCLOS).

The grand bargain of UNCLOS allowed states to claim as much as 12 miles from their coasts as “territorial sea” in which they could impose their own regulatory controls in return for committing to allow “innocent passage” of ships from anywhere, so long as they did not directly threaten the coastal state. States could also claim an “exclusive economic zone” as far as 200 miles from the coast, in which they could regulate fishing, mining, and polluting practices. Bosco presents this outcome as a major setback for the Grotian vision of freedom of the seas. He notes that unilateral national claims have exceeded controls authorized by UNCLOS and international conservation treaties have tried to add more restrictions on deep-sea fishing. The trend seems to be toward more and more control. Hence Bosco’s final word: “the future ocean will feature less traditional freedom than ever before. Grotius may have prevailed in the ‘battle of the books’ [between champions and opponents of maritime freedom] that played out in the 17th century—but he is on course to lose the war.”

This is certainly plausible. It may be correct. But Bosco’s long string of anecdotes, however suggestive and intriguing, does not establish this conclusion. The problem with the Poseidon Project is that it doesn’t offer much argument about the forces behind the trends, which could help clarify the trajectory of events and distinguish confirming (or contradicting) trends from random variation. It might be that we owe maritime freedom to the predominance of great trading states (the Netherlands, then Britain, later the United States) which happened to have the most powerful navies, as Bosco suggests. Or it might well be (as Captain Mahan argued) that a great trading nation is bound to build up naval strength to protect its trade.

The Poseidon Project doesn’t even offer much analysis about what Grotius meant or what we should still understand as “freedom of the seas.” So it’s hard to see the baseline against which to measure the trends. Thus, while the Allied navies exerted far-reaching control over world commerce during the world wars, the control was withdrawn when the wars ended. At the time, these measures were depicted as exceptional and temporary. Many decades later, they still look that way. Today, we can impose economic sanctions by denying access to banking networks, without needing to stop ships on the high seas. Or is it too optimistic to think this leaves no motive to interfere with ships on the high seas? European states have recently seized yachts of Putin-friendly oligarchs (or anyway, very wealthy Russians) on the high seas—because they could.

Whether war is an exception or the set of practices that reveal the deepest logic of statecraft is, of course, a question that goes beyond the law of the sea. So too with a related meta-question, regarding restrictions on freedom for the sake of freedom, as restrictions on disruptive protests are often defended as a necessary safeguard of controversial speech. To think about when restrictions on specific maritime practice may serve the larger end, we have to be more clear about the relevant ends. Bosco characteristically mentions the setting of Grotius’ great work on freedom of the seas, Mare Liberum (“Freedom of the Seas”). But he then rushes through the Grotian arguments in half a page, without much effort to analyze them.

Grotius gave two practical reasons why the seas should be open to all, both of which, in fact, have been rendered less compelling by modern technology. Grotius argued that seas could not be closed off by walls or fences, in the same way as land—an argument that seems far less compelling in the age of sea mines, submarine fleets, and air patrols. He also argued that the use of the sea by one nation left no one else worse off—an argument that is far less compelling when some fish species have been overfished to near extinction and oil spills and other pollutants threaten all manner of marine wildlife.

But Grotius also offered a moral argument, which might be called a theological argument. Grotius, himself, deployed the terminology of natural law: he spoke of a “self-evident” truth, proclaimed by “God Himself . . . speaking through the voice of Nature . . . ” God has scattered different natural resources in different places so that no people can be fully or comfortably self-sufficient: Our Creator seems to have arranged the world in this way, because He “wished human friendships to be engendered by mutual needs . . . ”

The echoes here—or rather, the anticipations—of formulas in the Declaration of Independence are not sheer coincidence. The argument about the seas is the logical complement to the Declaration’s claim for the “separate and equal station” of sovereign states: what does not belong to any one state should be open to all, since no one state (or limited coalition of states) can rightly claim to be master of the seas. Whether you find that claim compelling turns, perhaps, on whether you give any credit to any version of natural law reasoning.

Plainly the American Founders did. They cared about the law of the sea because colonial America depended on sea-borne trade (hence the protest in the Declaration against the British king “for cutting off our trade with all parts of the world”). But they also sensed the moral, perhaps even metaphysical stakes—hence Madison’s investment in classic treatises on natural law and the law of nations.

Elements of International Law, the first full-length treatise on “international law” in English (which helped popularize that new term, displacing “law of nations”) was written by Henry Wheaton, who had been court reporter for Chief Justice John Marshall. In a letter later published in Wheaton’s treatise, Marshall thanked him for having raised “Old Hugo Grotius . . . to the rank he deserves.”

The Poseidon Project lets readers glimpse why the arguments of Grotius were important to the American Founders and to generations of American statesmen thereafter. Though a lively survey in other respects, the book doesn’t get readers much past the threshold of understanding of why these older arguments have retained so much resonance for so long.