Suppose that Country A and Country B are having some sort of crisis, and Country A threatens to invade unless it gets its way. (A common occurrence, sadly.) But wars are costly, even if you win. Mobilizing your army diverts resources away from other critical activities, such as harvesting crops for the year. Battle casualties represent huge losses of human talent and labor. And every time you fight a war, you run the risk of losing. That all being the case, why would anyone decide to fight a war? And when?

That is, we have to explain four things: the choice of Country A to threaten war, the choice of Country B not to submit, the choice of either country to actually start the war, and the choice of the other country not to simply surrender and save itself the trouble of fighting.

There are many ways of explaining this sequence. But one powerful model to use, because it is so flexible and easily covers a whole range of situations, is the bargaining model of war developed by James Fearon and others in that vein.

The key variables of this model are:

• The cost of fighting, for each side;
• The total potential benefits of winning; and
• The likelihood of each side winning.

Essentially, if you know for a fact that you are likely to win, and that the benefits of winning exceed the cost of fighting, you are very likely to fight—and the other side is very likely to back down.

For example, suppose that you lead a company of 100 mercenaries, and you have the chance to attack a gold mine held by 30 opposing mercenaries. If you do, you expect to lose 15 of your troops, but you would gain the lucrative gold mine and you would very likely be able to keep it. Given that, chances are you’re going to attack the gold mine, even at the cost of some of your troops. And knowing this, the 30 mercenaries are likely to retreat or surrender before you attack, because it is pointless to fight and die when they know they would lose.

On the other hand, the losing mercenaries know that they could kill 15 of your troops if they do fight, and they know that you know it too. So they could negotiate with you for a settlement where they are allowed to take some amount of gold with them as they go—say, the equivalent of 10 mercenaries. So even though they would lose, the weaker side has an incentive to push for some share of the loot before they capitulate.

War thus becomes a bargaining process, where the two sides are essentially negotiating over how to split up the stakes of a war.

If so, why do people fight wars at all? Why not tally up opposing forces, figure out who would win and how much the net profit would be, negotiate some sort of settlement where the stronger party gets the same or greater profit and the weaker party is left with something, and avoid all the messy killing and burning and looting?

The most common reason is uncertainty. In the real world, it is often difficult to know who would win a war. It is also difficult to know how costly a war would be, and even what the benefits would be of winning. As a result, says the theory, any factor that increases uncertainty would tend to make war more likely, because each side hopes that it will end up being worth it to fight. And even if one side knows it would lose, the other side might be so overconfident that it asks for far too much of the “loot”; the weaker party may then decide to fight anyway, in hopes of keeping at least some of what it has.

And any factor that increases certainty would tend to discourage war. If the costs and benefits of war are better known, both parties will recognize when a war would be wasteful—or when the benefits of fighting are so obvious that the winning side cannot be deterred. And of course, if it is obvious who would win a war, the weaker side is likely to capitulate to save itself greater loss; the stronger side, too, is unlikely to demand too much, since it knows the point at which the other side would fight regardless. So, many conflicts would be avoided because the game is not worth the candle, and many others would end with the sides negotiating some sort of settlement, without fighting.

A second reason for war is if the “loot” cannot be split up between the sides. For example, in a war of extermination, there is simply no option of a settlement; you win, or you die. Less drastically, if two countries are fighting over control of a strategic mountain pass, there is no way to share the pass; one side is going to end up in control, and the other side will be shut out. So the stakes are higher, and there is less opportunity to negotiate a settlement.

Finally, what if you should be able to negotiate a settlement, but you don’t trust the other side to keep it? Or you can’t convince the other side that you can be trusted? Then it becomes much harder to negotiate an end to the crisis, and much easier (so to speak) to go to war instead. For example, rebel forces negotiating with a government have a very hard time coming to an agreement. Each side fears that any concessions will simply make the other side stronger; and often, the rebel forces are being supported by a rival country, which will sometimes pressure the rebels to keep fighting even when they want to stop (or vice versa).

That’s the basic model. It can be elaborated on in many ways, such as adding concerns about reputation or honor in a repeated game. (If you surrender once, maybe countries can bully you into submission over and over again in the future. So the long-term costs of surrender may end up being much higher that the immediate costs of fighting, all things considered, even if you know you will lose.)

So in your fiction, if you want to set up the conditions for a jolly old war, these are the key points to adjust: the cost of fighting, the prize for winning and whether it can be shared, the relative strength of the sides, the ability of each side to commit to a settlement, and the uncertainty of each side about any of the foregoing. A relatively simple model, and quite powerful—my favorite kind of writing tool!

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(This post is part of Politics for Worldbuilders, an occasional series. Many of the previous posts in this series eventually became grist for my handbook for authors and game designers, Beyond Kings and Princesses: Governments for Worldbuilders. I am now moving my attention to the planned second and third books in this series; the subject matter of this post fits into the third book, working title War for Worldbuilders. No idea when it will be finished, but it should be fun!)