Except for a few heroes or fanatics, nobody wants to be the last one to fight when his comrades (or perhaps foreign allies) have stopped shooting, abandoned their position, or surrendered. And every soldier knows that every one of his comrades is having the same thought about where his self-interest lies. So when they think the wind is about to turn, it has already turned and the whole battalion or army lays down its arms. This explains Afghanistan last week.

The prospect of 72 virgins in the afterlife counts of course, but more mundane incentives too.

Game theory has formalized this sort of problem as the famous Prisoners Dilemma. It may be in the common interest of all to continue fighting, but if every individual thinks it is in his own interest to stop, he will. It’s standard economics. In his book Bureaucracy (Liberty Fund, 2005), Gordon Tullock analyzed individual incentives in the military (which is a sort of bureaucracy) and gave numerous examples of their importance.