The question is often asked whether history can be ‘applied’, in other words, whether observation of the past can help solve current problems. The application of the past has figured prominently in the claims being made by Vladimir Putin about the origins of Russia, or the use of ancient maps to assert Chinese claims to the South China Sea. But another way of mobilising the past might be to see how the exercise of political power as long ago as the era of Roman domination over the entire Mediterranean basin helps us understand the exercise of hegemony, and whether other models can be found which preserve what might be called international order without the dominance of a single power. This is what Amitav Acharya and Manjeet S. Pardesi seek to do in Divergent Worlds, setting alongside one another the ancient Mediterranean and the ancient and medieval Indian Ocean. They are not a perfect match chronologically, because the Indian Ocean flourished while the Roman Empire waned; however, the contrast between a generally stable Indian Ocean region, in which no single power was dominant, and a Roman Mediterranean where peace was imposed by patrols of the Roman navy is very noticeable.

The question Acharya and Pardesi ask is timely, at a moment when the United States under Donald Trump is doubtful about its global role, and when China has rapidly built substantial naval forces and vast commercial fleets, leaving observers uncertain about its long-term ambitions. Although it possesses no Indian Ocean coastline, China features prominently in Divergent Worlds because the authors wisely follow the practice of thinking of an interconnected ‘Indo-Pacific’ region linking the Indian Ocean to the South China Sea. Although their omission of what was happening in the western Indian Ocean occasionally throws their account of the history of that ocean off balance, this intimate relationship between the waters off China and the Indian Ocean is observable over the last two thousand years.

Divergent Worlds is a well-organised book that begins by offering a coherent account of the relevant vocabulary of international relations. What is meant by ‘hegemony? What is meant by ‘international system’ or ‘Liberal International Order’? They are alert to the problem that the term ‘international’ may be inappropriate because it precedes the Westphalian conception of sovereignty articulated in 1648, often regarded as the foundation stone of international law. Their awareness of the importance of not just naval power but of maritime commercial networks gives the book unity. Most modern theoreticians emerge as less significant figures than the nineteenth-century American naval historian Alfred Thayer Mahan, whose observation of how British sea power was exercised, along with his knowledge of Roman antecedents, underlay the argument of The Influence of Sea-Power upon History 1600–1783, published in 1890. It was a guiding text not just for Theodore Roosevelt but for Kaiser Wilhelm and, in due course, for Soviet strategists of the Stalinist era such as Admiral Gorshkov.

The core of Divergent Worlds consists of two essays, one concentrating on the hegemonic Roman Empire and the other on the very different scenario in maritime south-east Asia. Parts of the book consist of a stream of quotations taken not from original sources but from modern writers (including myself). The Roman chapter in particular is a disconcerting forest of quotation marks. Many of these quotations are expressions of opinion, rather than hard facts, and more attention to empirical evidence is often required, so as to avoid the danger of creating a tottering tower of assumptions and contentions that does not correspond to historical reality. However, the authors make some very germane points. One concerns the way control of the seas was exercised. The suppression of piracy, beginning with the campaigns of Pompey in the first century BC, enabled the creation of increasingly important trade routes carrying grain from Egypt, Tunisia and elsewhere to Rome. A bipolar space was created, with Rome and Alexandria at each end, to which later the ‘New Rome’, Constantinople, would be added. The Roman navy, with its bases at Misenum, near Naples, and Classis, near Ravenna, was on hand to deal with any emergencies. On the other hand, they might have noted that service in the navy was not regarded as comparably prestigious to service in the army. The navy was seen as the soft option, precisely because it had been so successful in establishing a Pax Romana across the Mediterranean.

The Indo-Pacific, as they rightly aver, functioned very differently, lacking a single hegemon; the arrival of merchants from the Roman Empire (mainly Greeks from Egypt) in India did nothing to change the situation. Plenty of petty rulers took charge of local trade—they might have mentioned the kings of Kollam in southern India, who in the mid-ninth century extended their protection over merchants of many faiths and ethnicities trading both westwards and eastwards. Even the so-called empire of Sri Vijaya, in the early Middle Ages, only exercised regional power from its base at Palembang in Sumatra; and the extent to which it controlled the passage through the Strait of Malacca is uncertain. Li Tana’s recent A Maritime Vietnam (2024) presumably appeared too late to be cited, but it adds an important Vietnamese dimension to Divergent Worlds, showing, for instance, how Vietnam filled the gap left by the shrinking of China’s maritime trade in porcelain during the fifteenth century. However, the overall stability of this maritime space over many centuries is striking, even allowing for Chola attacks from South India on Sri Vijaya. The result of this stability was a constant flow of merchants and their goods, dependent on the monsoon winds—the ‘Silk Road of the Sea’, as it is nowadays called, linking the Indian Ocean not just to the Spice Islands but to China, with its silks and ceramics. It is a pity that Acharya and Pardesi do not delve into the history of some of the key ports such as Singapore, already a major centre of trade in the fourteenth century, and its successor further along the Strait of Malacca, Melaka, an extremely successful trading hub that came under Chinese influence around the time of the famous Ming voyages led by Admiral Zheng He (1405–1433). Their foundation and efflorescence confirm the sense that dynamic rulers of newly founded ports could achieve remarkable success through fostering trade.

China is, though, the elephant in the room, as the final chapter demonstrates. Although, as the authors show, it did not extend its power into the Indian Ocean—Zheng He’s voyages were an opportunity to parade rather than impose Chinese power—its looming presence cannot be ignored. Setting aside the Southern Song and Yuan periods (c.1090–1368), China has generally looked away from the sea. Formally, at least, the Chinese emperors did not encourage trade but received lavish tribute and sent handsome gifts in return. Today, though, we are looking at a revival of Chinese maritime ambitions, linked both to the Belt and Road Initiative and to the domination of the South China Sea, including Taiwan. What emerges from this book is that China tends to watch and wait. The current calm in the Indian Ocean may well turn into choppy seas in the face of attempts by Trump, Putin, and possibly Xi to create a new world order. This intriguing, stimulating and original book will help us think about the different ways in which stability without hegemony can flourish.