Whenever governments debate whether to keep trading with a rival power, an old assumption comes up. Trade, the argument goes, binds nations together. Countries that buy and sell from each other have too much to lose by fighting. Germany's post-Cold War foreign policy even gave the idea a slogan: Wandel durch Handel, or "change through trade."

The idea did not begin with economists or diplomats. It came from Enlightenment philosophers who believed commerce could do what kings and treaties had failed to do. They hoped trade would make peace a matter of everyday self-interest rather than fragile promises. Their argument, and the places where it broke down, help explain why we still place so much hope in markets.

In 1748, Montesquieu published The Spirit of the Laws, a sweeping study of how climate, religion, and economics shape governments. In it he made a striking observation: it is almost a general rule that wherever there is commerce, manners are gentle, and wherever manners are gentle, there is commerce.

Scholars call this the doux commerce thesis, meaning "gentle commerce." The reasoning was as much psychological as economic. Merchants must travel, negotiate, and deal with strangers. They learn the customs of other peoples. Over time they come to value reputation, reliability, and calm calculation more than honour, glory, and revenge.

This reversed centuries of moral thinking. Trade had long been seen as grubby and suspect, while martial courage was celebrated as noble. Montesquieu admitted that commerce could corrupt purer morals, but he argued that it also polished rough and violent ones. The economist Albert Hirschman later described this outlook as the hope that steady interests could tame dangerous passions. The aim was not to make people saints. It was to give them reasons to behave predictably.

Takeaway

Montesquieu's insight was that the way people earn a living quietly shapes their character. Institutions teach virtues, often without anyone noticing the lesson.

Montesquieu also offered a more practical argument. When two nations trade, he wrote, they become reciprocally dependent. If one has an interest in buying, the other has an interest in selling. Peace, in his words, is the natural effect of trade.

Other thinkers built on this foundation. David Hume, in his essay "Of the Jealousy of Trade," attacked the mercantilist belief that one nation's wealth must come at another's expense. A prosperous neighbour, he argued, was a better customer, not a threat. Adam Smith extended the case in The Wealth of Nations (1776), showing that voluntary exchange could benefit both sides at once. Trade was not a battlefield with winners and losers.

Immanuel Kant drew the political conclusion in Perpetual Peace (1795). The "spirit of commerce," he wrote, cannot coexist with war, and sooner or later it takes hold of every nation. This was a quiet revolution in thinking about peace. It no longer depended on the good faith of rulers. It rested on something sturdier: the calculations of millions of ordinary people whose livelihoods would be ruined by conflict.

Takeaway

A treaty asks parties to keep a promise, while interdependence makes breaking that promise expensive. The Enlightenment bet that cost is a more reliable guardian of peace than virtue.

By the early twentieth century, the theory seemed confirmed. Britain and Germany were among each other's largest trading partners. In The Great Illusion (1910), Norman Angell argued that conquest had become economically pointless in an interconnected world. Victors, he said, would gain little and lose a great deal.

In 1914, Europe went to war anyway. The commercial argument assumed that nations behave like merchants, weighing costs against benefits. Nationalism spoke a different language, one of honour, identity, and destiny. Millions proved willing to sacrifice prosperity for causes they believed were larger than themselves. Angell had been right about the costs. He had been wrong to think costs would decide the matter.

Dependence also cut both ways. Hirschman later showed how Nazi Germany used trade to tie smaller countries in southeastern Europe to its will. Interdependence can become leverage, and leverage can become coercion. When trade is unequal, it may breed resentment rather than goodwill. Commerce changes the incentives people face, but it cannot control how they interpret them, or what they are willing to give up.

Takeaway

Rational interest is a powerful force, but it is not the only one. Any theory of peace that ignores pride, fear, and identity describes people as we might wish them to be.

The Enlightenment thinkers were less naive than their critics suggest. Kant paired commerce with republican government and a federation of free states. Trade was one pillar, not the whole structure. Europe's postwar project, beginning with shared coal and steel, combined economic ties with binding institutions.

Today's debates about sanctions, supply chains, and rival powers revisit the same question. Trade can make peace more likely. Whether it makes peace secure depends on what we build around it.