War is costly in lives, money and risk, and both sides usually know it. That makes it a puzzle. If the two sides could foresee roughly how a war would end, why not agree to that outcome without paying for the fighting?

War as politics

The Prussian general Carl von Clausewitz gave the classic starting point in On War, published after his death in 1832. In J. J. Graham's 1873 translation, "War is a mere continuation of policy by other means." Clausewitz meant that war serves political aims and should be judged by them; it is not a separate realm with goals of its own. He also stressed what he called friction. Friction, for Clausewitz, is the mass of small difficulties that make real war unpredictable: orders misunderstood, roads turned to mud, rumors taken for facts. Plans that look simple on paper rarely survive contact with it.

The bargaining puzzle

In 1995 the political scientist James Fearon sharpened this into a puzzle. Since war destroys value, there should almost always be some peaceful deal that both sides prefer to fighting. So why does war happen? Fearon offered three answers. The first is private information combined with incentives to misrepresent it. Each side knows its own strength and resolve better than its opponent does, and each has reason to bluff, so the two may honestly disagree about who would win. The second is the commitment problem. A commitment problem arises when one side cannot credibly promise not to exploit a future advantage; a state that expects to decline may prefer war now to a worse bargain later. The third is indivisibility, when the thing in dispute cannot be split, such as a sacred site, though Fearon thought true indivisibility was rarer than it looks.

The Australian historian Geoffrey Blainey had reached a related conclusion in The Causes of War (1973). Blainey argued that wars tend to begin when states disagree about their relative strength, and end when the fighting has settled that disagreement. War, in this view, is partly a brutal way of discovering information that could not be credibly shared.

Misperception and domestic politics

Not every error is a rational one. Robert Jervis showed in Perception and Misperception in International Politics (1976) that leaders tend to fit new evidence into existing beliefs and to see rivals as more hostile and more unified than they really are. Domestic politics matters too. Diversionary war theory holds that leaders in trouble at home may start conflicts abroad to rally public support. The evidence for it is mixed: some embattled leaders gamble on quick victories, but many become more cautious, since a lost war can end a regime.

How much war is there?

The Correlates of War project, founded at the University of Michigan by J. David Singer in 1963, built the datasets most scholars use. The Correlates of War project counts a conflict as a war when it causes at least 1,000 battle deaths. Its data and others show a broad pattern since 1945. Wars between states have become rare, while civil wars, often with foreign involvement, make up most of the fighting. Conquest of territory has also become much less common than in earlier centuries.

Whether this marks a lasting change is disputed. Steven Pinker argued in 2011 that violence of many kinds has declined over the long run. Critics such as Bear Braumoeller replied in 2019 that the rate at which states start conflicts has not clearly fallen, and that the largest wars are so rare that a few decades of relative calm may not reveal a real trend. Russia's invasion of Ukraine in 2022 reopened the argument.

Worked example: the Falklands War, 1982

On 2 April 1982 Argentina's military government invaded the Falkland Islands, a British territory in the South Atlantic that Argentina calls the Malvinas and had long claimed. Britain sent a naval task force on a voyage of roughly 13,000 kilometers. Argentine forces on the islands surrendered on 14 June. About 650 Argentine and 255 British servicemen died, along with three islanders.

Each explanation from this level finds something. The junta, led by General Leopoldo Galtieri, faced economic crisis and growing street protests, which fits diversionary war theory. It also misjudged British resolve. Britain had announced the withdrawal of its only naval patrol ship in the area, which Argentine leaders read as a sign that it would not fight. That is the problem of private information in Fearon's terms: neither side could easily prove how much it cared. American mediation failed partly because, once the islands were taken and the first lives lost, neither government could accept a compromise without paying a heavy price at home. Blainey's view fits the ending. Once the fighting had shown which side was stronger, the war ended quickly. A war that looks irrational from outside turns out to be built from choices that seemed sensible, or at least survivable, to the people who made them.