The most notorious aphorism of the Middle Ages is probably the comment of the papal legate Arnauld-Amaury during the sack of Beziers in July 1209, “God will know his own. Kill them all.” This was the first city to fall to the Albigensian crusaders and it certainly contained many more Catholics than it did Cathars. There has been some debate as to whether Arnauld-Amaury actually said the famous words, but general agreement that he would not have disagreed with the sentiment. After all, the Catholics had been sheltering the heretics, which was quite bad enough.
There is one important point to grasp about the remark, however. Arnauld-Amaury was not issuing an order. Beziers had fallen by storm and its population were going to get slaughtered as surely as night follows day. The papal legate was merely giving his stamp of approval to something that was going to happen anyway. Why would he do that? Partly, it was because he had to keep the army on side. He couldn’t afford to deny the crusaders, who were a volunteer force, the spoils of war. But mainly, I think, it was because he understood and approved of the medieval rules of war.
The rules were really quite simple. If a town or castle surrendered, then the terms of their surrender would be regarded as sacrosanct. If, however, the attacking army took their target by assault, then there would be no mercy. Again and again, even during the Albigensian Crusade, garrisons could march out of a castle that they surrendered while anywhere that refused to capitulate would, when it eventually fell, be reduced to a smoking ruin. Clearly, in the long term, this worked to everyone’s advantage. If you want your opponents to surrender, then you have to ensure that the pay off for doing so is as great as possible. Thus, you cannot trick them into surrendering and then slaughter them, because no one would ever believe you again. You would have to besiege or assault every village you came to and thus never get very far. Likewise, if you let the population be after they have held out against you with all their might, you send out the message that others have nothing to loose by resisting.
This is why the slaughter after the fall of Jerusalem was not the great atrocity that we commonly believe, but simply par for the medieval course. Even then, the Arab garrison who shut themselves in the castle were able to negotiate their safe surrender. In fact, what I find so surprising was how often medieval garrisons were allowed to retire unmolested and how completely safe conduct was respected.
Comments or questions? Post them at Bede's dedicated yahoo group.