Want world peace? Support free trade.

Everyone knows that a key to the Democrats' big electoral win was their opposition to the Iraq war. But also, as the Wall Street Journal reported recently, "Democrats' stances against free trade helped build the party's success at the polls and could tip the balance on trade matters. The new dynamic could put a definitive end to the already troubled effort to reach a global agreement to reduce tariffs and open markets...."

Protectionists (of whatever party) believe that consumers who buy goods and services from foreigners cause domestic employment – and wages – to fall. Economists since before Adam Smith have shown that this belief is mistaken, largely because foreigners sell things to us only because they either want to buy things from us or invest in our economy.

These activities employ workers here at home and raise their wages. Mountains of empirical evidence show that protectionism is economically destructive. The facts also show that protectionism is inconsistent with a desire for peace – a desire admirably expressed by many Democrats during the recent campaigns.

Back in 1748, Baron de Montesquieu observed that "Peace is the natural effect of trade. Two nations who differ with each other become reciprocally dependent; for if one has an interest in buying, the other has an interest in selling; and thus their union is founded on their mutual necessities."

If Mr. Montesquieu is correct that trade promotes peace, then protectionism – a retreat from open trade – raises the chances of war.

Plenty of empirical evidence confirms the wisdom of Montesquieu's insight: Trade does indeed promote peace.

During the past 30 years, Solomon Polachek, an economist at the State University of New York at Binghamton, has researched the relationship between trade and peace. In his most recent paper on the topic, he and co-author Carlos Seiglie of Rutgers University review the massive amount of research on trade, war, and peace.

They find that "the overwhelming evidence indicates that trade reduces conflict." Likewise for foreign investment. The greater the amounts that foreigners invest in the United States, or the more that Americans invest abroad, the lower is the likelihood of war between America and those countries with which it has investment relationships.

Professors Polachek and Seiglie conclude that, "The policy implication of our finding is that further international cooperation in reducing barriers to both trade and capital flows can promote a more peaceful world."

Columbia University political scientist Erik Gartzke reaches a similar but more general conclusion: Peace is fostered by economic freedom. Economic freedom certainly includes, but is broader than, the freedom of ordinary people to trade internationally. It includes also low and transparent rates of taxation, the easy ability of entrepreneurs to start new businesses, the lightness of regulations on labor, product, and credit markets, ready access to sound money, and other factors that encourage the allocation of resources by markets rather than by government officials.

Professor Gartzke ranks countries on an economic-freedom index from 1 to 10, with 1 being very unfree and 10 being very free. He then examines military conflicts from 1816 through 2000. His findings are powerful: Countries that rank lowest on an economic-freedom index – with scores of 2 or less – are 14 times more likely to be involved in military conflicts than are countries whose people enjoy significant economic freedom (that is, countries with scores of 8 or higher).

Also important, the findings of Polachek and Gartzke improve our understanding of the long-recognized reluctance of democratic nations to wage war against one another. These scholars argue that the so-called democratic peace is really the capitalist peace.

Democratic institutions are heavily concentrated in countries that also have strong protections for private property rights, openness to foreign commerce, and other features broadly consistent with capitalism. That's why the observation that any two democracies are quite unlikely to go to war against each other might reflect the consequences of capitalism more than democracy.

And that's just what the data show. Polachek and Seiglie find that openness to trade is much more effective at encouraging peace than is democracy per se. Similarly, Gartzke discovered that, "When measures of both economic freedom and democracy are included in a statistical study, economic freedom is about 50 times more effective than democracy in diminishing violent conflict."

These findings make sense. By promoting prosperity, economic freedom gives ordinary people a large stake in peace.

This prosperity is threatened during wartime. War almost always gives government more control over resources and imposes the burdens of higher taxes, higher inflation, and other disruptions of the everyday commercial relationships that support prosperity.

When commerce reaches across political borders, the peace-promoting effects of economic freedom intensify. Why? It's bad for the bottom line to shoot your customers or your suppliers, so the more you trade with foreigners the less likely you are to seek, or even to tolerate, harm to these foreigners.

Senators-elect Sherrod Brown (D) of Ohio and Jim Webb (D) of Virginia probably don't realize it, but by endorsing trade protection, they actually work against the long-run prospects for peace that they so fervently desire.

• Donald J. Boudreaux is chairman of the economics department at George Mason University.