Trade, cheap shipping and global finance have tied countries' fates together more tightly than ever. Does that make peace more likely? And can a system of separate sovereign states handle problems, such as a warming climate or a new virus, that respect no border?

Trade and peace

Commercial liberals have argued since the eighteenth century that trade makes war less attractive, and the English politician Richard Cobden campaigned in the 1840s for free trade as a force for peace. In The Great Illusion (1910), the journalist Norman Angell argued that in an economically interdependent world conquest no longer paid, since a victor would wreck the very trade and credit its own wealth depended on. Angell is often mocked because the First World War broke out four years later. But Angell's claim was that war had become economically ruinous, not that it had become impossible, and the war's cost arguably proved him right.

Robert Keohane and Joseph Nye developed the idea of complex interdependence in Power and Interdependence (1977). Complex interdependence describes ties among states through many channels, with no fixed ranking of issues and little use for military force between them. It fits relations between, say, Canada and the United States, where disputes over lumber or dairy are fought by lawyers and negotiators, not armies. Keohane and Nye insisted, though, that interdependence is rarely equal.

Interdependence as a weapon

That warning goes back to Albert Hirschman's National Power and the Structure of Foreign Trade (1945), which studied how Nazi Germany built trade ties with smaller states in southeastern Europe. Hirschman showed that when trade matters more to one partner than to the other, the less dependent partner gains power, since it can threaten to cut ties at little cost to itself. In 2019 Henry Farrell and Abraham Newman extended the argument to modern networks. Weaponized interdependence is the use of control over central hubs in global networks, such as payment systems, to watch or coerce others. The SWIFT network for bank payment messages, based in Belgium, cut off Iranian banks in 2012 and several major Russian banks in 2022. Many governments now treat heavy dependence on a rival for energy, computer chips or rare minerals as a security risk.

The evidence on trade and peace is therefore mixed. Many studies find that states that trade heavily with each other fight less often. Yet Britain and Germany were major trading partners in 1914, and Europe's large purchases of Russian gas did not prevent the invasion of Ukraine.

Global problems and collective action

Some problems require many states to act together. The economist Scott Barrett classified such problems by how individual efforts add up. For climate change, what counts is the total of all countries' efforts, so each state is tempted to let others cut emissions while it rides free. Disease eradication is a weakest-link problem: if one country fails, the disease survives and can spread back everywhere.

Some cooperation has worked well. The Montreal Protocol of 1987 phased out ozone-destroying chemicals, helped by trade limits on non-members and financial aid for poorer countries, and the ozone layer is now slowly recovering. Pandemics have gone less well. Under the International Health Regulations, revised in 2005, states must report outbreaks that may spread abroad, but the World Health Organization has no power to force them to. During COVID-19, rich countries bought most of the early vaccine doses, and the shared purchasing scheme COVAX fell well short of its targets. In 2025 WHO members adopted a pandemic agreement, though key details on sharing pathogens and vaccines were left for later talks.

Worked example: eradicating smallpox

Smallpox killed perhaps 300 million people in the twentieth century alone. In 1958 Viktor Zhdanov, a Soviet deputy minister of health, proposed to the World Health Assembly that the world eradicate it. The WHO launched an intensified campaign in 1967, directed by the American epidemiologist D. A. Henderson, with both superpowers contributing vaccine and expertise at the height of the Cold War. Rather than try to vaccinate everyone, teams used surveillance and containment: find each case quickly, then vaccinate everyone around it. The last natural case occurred in Somalia in 1977, and in 1980 the World Health Assembly declared smallpox eradicated.

The case shows how a weakest-link problem can be solved. Every country benefited, the goal was clear and could be checked, and rich countries had a direct interest in helping the poorest finish the job, since any surviving pocket threatened them too. Rivals cooperated because the gain was shared and the cost of helping was modest. Climate change is harder: the benefits of cutting emissions are spread thinly across the whole world and over generations, while the costs fall on each state now. Smallpox is not a model to copy blindly, but it is a reminder that a world of sovereign states, for all its anarchy, can protect human life on a planetary scale when the incentives line up and people choose to act.