What Wars Really Start Over

The shooting in Sarajevo on June 28, 1914, created neither alliances, nor armies, nor ready-made mobilization plans - all of that was already on the table. It gave Austria-Hungary what it lacked: a legitimate pretext to issue Serbia an ultimatum right then. Remove the shooting, and the alliances, armies, and plans remain; historians debate something else: whether another pretext would soon have emerged or the crisis would have had time to cool down.

That is why historians almost always separate two things. A pretext is the event after which people start counting days. A cause is what made people respond to that event in that particular way. When Thucydides analyzed the war between Athens and Sparta, he too separated the stated grievances from what he saw as the real cause: Athens was growing stronger, and Sparta feared it.

Political scientists see war as negotiations that failed to happen. The logic is simple: war is costly for both sides, so there is almost always some way to divide what is disputed - land, influence - that both would prefer to fighting. If such an option exists but the sides still go to war, they failed to reach it.

Two things get in the way. First, in negotiations it pays to exaggerate your resolve and hide your weak spots, so no one knows whose bluff is real. The only way to test it is by force. Second, future promises are worth nothing if your neighbor is rapidly growing stronger. Its promise today not to attack will, in 10 years, come from a different, far more powerful country. Then the side that is stronger for now but losing ground starts to think the right moment is now, and things will only get worse later.

This leads to a surprising conclusion: the overwhelming majority of crises that looked like they could lead to war did not end in war. Fashoda in 1898, Morocco in 1911 - tough demands, fleets, bellicose newspapers, and then the sides simply went their separate ways. The usual shape of a crisis is a rise and a fall.

War happens where the decline never comes. And often because both sides misjudge the cost: in 1914, expectations of a short war were widespread, and many expected to be home by Christmas. This is a model, not a law - historians debate how well it explains each particular war, and in some cases domestic politics, not calculations about a neighbor, prove decisive.