Perpetual Peace
Kant's 1795 essay arguing that republics would not fight each other; the intellectual root of liberal internationalism and modern democratic peace theory.
In 1795 a seventy-one-year-old Prussian philosopher who rarely left the city of Konigsberg published a slim essay laid out like a peace treaty. Zum ewigen Frieden, "Perpetual Peace: A Philosophical Sketch," was Immanuel Kant's answer to a brutal century of dynastic war: a claim that lasting peace was not a fantasy for angels but an achievable arrangement among self-interested states, if they were built the right way. Two centuries later it is the founding charter of liberal internationalism and the ancestor of the closest thing political science has to an empirical law.
Kant's blueprint
Kant wrote the essay as a mock treaty, with "preliminary articles" (abolish standing armies, contract no debts for war, never interfere by force in another state's constitution) and three "definitive articles" that carry the argument. First, the civil constitution of every state should be republican: representative, with powers separated, so that those who must pay for a war in blood and taxes have a say in declaring it. Second, the law of nations should rest on a federation of free states, a voluntary league rather than a world empire. Third, a cosmopolitan right limited to universal hospitality, the guarantee that a stranger arriving on foreign soil not be treated as an enemy. Republics restrain war, Kant argued, because citizens will not lightly vote themselves into the slaughter that kings order so freely.
From essay to law
Kant claimed a tendency, not a guarantee. It fell to the American scholar Michael Doyle, in a pair of 1983 articles titled "Kant, Liberal Legacies, and Foreign Affairs," to turn the intuition into a testable finding. Doyle noticed that liberal democracies, whatever their quarrels with everyone else, almost never go to war with one another. That regularity, the democratic peace, is about as robust as anything in international relations, which is why one prominent scholar called it the nearest thing the field has to an empirical law. It is also the intellectual root of Woodrow Wilson's crusade to make the world safe for democracy and of the institutions, from the United Nations to the European Union, built after 1945 on the hope that Kant was right.
The fine print
The claim survives partly by being narrow. The peace is dyadic, not monadic: democracies rarely fight each other, but they are not measurably more peaceful toward everyone, and they have fought plenty of wars against non-democracies. Critics press on the definitions. How democratic must a state be to count, and how far back does the record run? Some of the peace may be a selection effect: democracies that trade heavily and share alliances have reasons beyond their constitutions to avoid war. And the historical sample is short enough that a single war between two mature democracies could dent the whole edifice. Still, more than two centuries after a philosopher sketched it as an ideal, the pattern he predicted has stubbornly refused to break. The debate is no longer whether the democratic peace exists, but exactly why.