The Duke of Wellington, surveying his soldiers before the Battle of Waterloo, famously said that he did not know what they did to the enemy, but by God they frightened him.
No one thought in those days of the psychological effect upon the soldiers of witnessing so much violence (more than 30,000 were killed during the battle, about one in six of those who took part in it); nor could anyone have done so if he had thought of it. But it is now accepted wisdom that active military service leads men subsequently to commit crimes of violence, though the reasons for this are unknown.
A recent paper in The Lancet examined the association of military service and subsequent crimes of violence, which turned out to be much weaker than suspected. The authors examined the criminal records of 8,280 British soldiers who had served in Iraq and Afghanistan with that of 4,080 of those who had not. When controlled for such factors as age, level of education, pre-service record of violent offenses, rank, and length of service, there was no significant difference in the criminal records of those who had served in Iraq and Afghanistan and those who had not.
When, however, those who were deployed in a combat role were compared with those who had not been so deployed, it was found that the former had higher levels of violent offending as measured by their criminal records. Interestingly, however, those who were involved in actual fighting had considerably higher prior levels of violent offending than those not so involved, suggesting that more aggressive types either volunteered or were selected for combat service. Somewhat alarmingly, nearly half of soldiers involved in the fighting had criminal records for violence.