Peace: political peace (democracies don't fight)

Source: Kant and the republican vision: when the people decide, peace wins

"The civil constitution in every state must be republican... The republican constitution offers the prospect for the desired peace... for if the consent of the citizens is required to decide 'whether there shall be war or not', nothing is more natural than that they will consider carefully before taking upon themselves such a terrible game... In contrast, in a constitution that is not republican, war is the easiest thing in the world, since the ruler is not a citizen of the state but its owner."

(Immanuel Kant, Perpetual Peace (1795), Second Section, First Definitive Article)

Explanation: Democratic peace: why those who pay the price don't rush to war

German philosopher Immanuel Kant introduced a revolutionary idea that remains relevant to this day (in political science, it's known as the "democratic peace"). The argument: kings and tyrants go to war easily because they don't pay the price; they stay in the palace while the citizens die. But in a "republican" state (what we today call a liberal democracy), where the people decide, the citizens won't rush to vote for war because they are the ones who will have to fight and pay the taxes for it.

Perspective: From the Mishnah to Kant: 1,500 years of wisdom on checks and balances

For the most part, history has proven Kant right: democracies almost never go to war with one another, resolving disputes through diplomacy instead (the foundation of the modern European Union). Kant wasn't naive; he didn't think human beings were angels, but believed in building a system of checks and balances that would curb the ruler's whims.

This concept was anticipated 1,500 years earlier by the Jewish sages, who established in the Mishnah (Sanhedrin 2:4) that a king is not permitted to launch an initiated war (milchemet ha-reshut, an optional war) without the approval of the Sanhedrin. Both Kant and the Mishnah understand: to prevent unnecessary wars, the decision must not be left in the hands of a single ruler, but handed over to the judgment of the public's representatives and reason.

Going Deeper: When goods cross borders, soldiers stay home: the economic dimension of peace

Frédéric Bastiat, the 19th-century French economist, added the economic dimension with the quote attributed to him: "When goods don't cross borders, soldiers will." The idea is that free trade creates mutual dependence. If I buy wheat from you and you buy wine from me, neither of us gains by fighting and destroying the market. Modern peace is often grounded in the cold economic self-interest of global corporations just as much as in moral ideals.

Culture: The only winning move: when a supercomputer learns not to play at war

The movie WarGames (1983) features a supercomputer that is given control over nuclear missiles and learns Kant's theory the hard way, but in reverse. At the end of the film, the computer runs millions of nuclear war simulations and reaches a conclusion: "A strange game. The only winning move is not to play." The takeaway is that in the modern era, war has become rationally unprofitable for any side, making peace (or at least the absence of war) the only logical choice.