When an American President or a British Prime Minister sends in the troops to some poor and unhappy country, he or she knows for sure that some of them will loot, torture, rape and kill. Not many, but enough to turn local hearts and minds decisively against the occupying forces.
Nowadays, we are supposed to see such things as a failure of discipline. But in the past, they were often the sanctioned reward for discipline. If you fought as you were told to fight, did not run away and were victorious - then often enough your reward was a temporary licence to do as you pleased: to loot shops and homes, to rape women, and to kill anyone who displeased you.
Occasionally, the whole purpose of a military intervention was to terrorise a local population. Aggrieved by their defeat by Afghan forces during the first Afghan War in the 1830s and 1840s, Britain sent in an "Army of Retribution". It was called that and that was its task, carried out with calculating malice - troops even pausing to cut rings in the bark of fruit trees to destroy Afghan orchards.
In the Crimean War, permission to loot was given both as a reward for successful campaigning and to relieve the boredom and frustration of British troops as they camped out in uncomfortable conditions.
That the First World War involved terrorising civilian populations was a fact half-known to my generation of British school boys who all knew the words of the song "Three German Officers crossed the Rhine".
At the end of the Second World War, as is now widely known, Red Army troops terrorised German women on their march to Berlin, raping tens of thousands and killing thousands. Stalin knew and approved: in his opinion, the men must have some reward.
But much less well known is the fact that Free French troops did the same as they entered Germany from the West, not on such a large scale but causing real terror in a few towns (Freudenstadt is the one most often mentioned) where they paused to enjoy themselves. American GIs also raped, though not in the same organised fashion.
Nowadays, we are accustomed to reading about the crimes of British and American troops in Iraq and Afghanistan. Those crimes are rooted in the the way we recruit, train and deploy our troops.
Military discipline aims to ensure that troops do as they are told. It is never wholly successful - all military histories are full of accounts of troops running away and even modern armies have detachments at the rear to catch and punish them. And doing as you are told does not mean the same as doing it willingly. Troops under pressure are often tired, stressed and frightened. American troops cope by using drugs and British ones by using alcohol. Both sometimes relieve the pressure by abusing the natives even when they have not been ordered to do so.