The study of almost 14,000 British soldiers deployed to Iraq and Afghanistan is the first to examine the link between military service and violent crime by using official criminal records.
Researchers said the findings could help military officials improve their risk assessment of violence among serving and ex-military personnel.
They stressed that although the study points to a serious problem for those affected, it does not mean all ex-soldiers will become violent criminals.
"Overall you must remember that of those who serve in combat, 94 percent of those who come back will not offend."
The study found that those in combat roles were more than 50 percent more likely than those in non-combat roles to commit assaults or threaten violence after returning.
The problem was particularly striking among young men. Of around 3,000 soldiers aged under 30, more than 20 percent had a conviction for violent offences, compared with only 6.7 percent of civilian men in the same age group.
The study also highlights mental health problems in the military, and issues of alcohol abuse, post-traumatic stress disorder and aggressive behaviour, the researchers said.
Violent offending was most common among young men from the lower ranks of the army, said Deirdre MacManus from King's College London, who led the work and presented the results at a briefing in London.
This behaviour was strongly associated with a history of violent offending before joining the military, she said.
The study's publication, in the Lancet medical journal on Friday, comes as military chiefs in the United States say a soldier charged with slaying 16 civilians in Afghanistan last year should undergo a sanity review.
Anecdotal evidence and media coverage of violence and assaults committed by ex-servicemen has focused attention on whether serving in combat makes soldiers less stable and more prone to violent outbursts.
The study's results found that men who had seen combat in Iraq and Afghanistan were 53 percent more likely to commit violent offences than their fellow soldiers in non-combat roles.
Men who had multiple traumatic combat experiences had a 70 to 80 percent higher risk of becoming violent criminals.
David Forbes, an expert in post-traumatic mental health from the University of Melbourne, Australia, said this study showed for the first time the link between combat and interpersonal violence, and the need for better understanding of the mechanisms behind how combat enhances the risk of violence.
"By understanding these factors, we might develop more informed prevention and intervention programmes for troops as they reintegrate into civilian life," he wrote in a commentary.
Wessely said that having naturally higher levels of aggression was likely to be an attribute for many soldiers.
"Some people with aggressive dispositions make very good soldiers, that's the nature of the game," he said.
