Younger members of the armed forces returning from duty are more likely to commit violent offences than the rest of the population, a study suggests.
Researchers analysed data from nearly 14,000 UK service personnel who had served in wars in Iraq or Afghanistan.
They highlighted a particular issue in younger men and those who had combat roles or had a traumatic experience.
The results in the Lancet medical journal come 10 years after the start of the war in Iraq.
The Ministry of Defence, which funded the study, said it was committed to improving services and trying to break the stigma around mental health by getting people in the armed forces and veterans to talk about their problems.
The report showed that overall criminal activity was slightly lower in military personnel than in people of the same age in the wider population. Some 94% of men returning from combat zones will not offend.
However, the researchers found violent offending was higher within members of the armed services and there was a "stark" difference in men aged under 30.
Just over 20% of the 2,728 young men followed had committed a violent offence, compared with 6.7% of young men outside the military.
Most violent offences were assaults.
Being in the junior ranks, deployment in a combat role and experiencing traumatic events, such as being shot at, were all linked to an increased risk of violence when service personnel returned from duty.
Alcohol abuse and post-traumatic stress disorder were also closely associated with violent behaviour.
The researchers did adjust their analysis to account for the backgrounds of those studied - those with a greater tendency towards violence may be more inclined to choose combat roles.
Prof Simon Wessely, from King's College London, told the BBC: "Those who are in combat roles are themselves slightly different from those who are not.
"The military don't select chess-playing choir boys. They select people who often come from difficult and aggressive backgrounds and they're the ones who are most likely to end up in the parts of the military that do the actual fighting.
"The biggest single risk factor is those who previously had violent offending before they joined up, but there is still an impact of combat, mediated partly through excessive drinking and partly through developing post-traumatic stress disorder (PTSD) and mental health problems as a result of combat."
He added that the reluctance of young men to admit they may not be coping is the "biggest single obstacle" to tackling the problem.
'Body and mind'
Lewis McKay, who served in Iraq and Afghanistan, said his character changed from "laid-back" to "aggressive" after he returned from Afghanistan.
"Nothing will ever prepare you for what you physically see or physically do in Afghanistan... there is only so much that anyone's body and mind can take.
"My wife didn't exist to me... I felt a lot of anger towards her and I came very, very close to hitting her.
"Instead I was walking out the door and punching holes in doors and windows... I had flashbacks. A car door slam would be enough to make me drop to the ground."
The 26-year-old added that until soldiers receive help, problems such as PTSD will continue to manifest. He now works as a security guard at a BBC building.
Surgeon Capt John Sharpley, a Ministry of Defence mental healthcare expert, agreed that getting young soldiers to ask for help was a "major issue".
"Stigma is a really big problem. The study shows there is a link between mental health symptoms and violent offending.
"It is not possible to train yourself for something that is traumatic, which by definition is something outside one's experience.
"We do a lot [at the MoD], but we're always going to be in a situation when we need to do more."
A spokesman for the MoD said: "We are committed to supporting members of our armed forces, and their families, as they return to civilian life post-deployment.
"This report recognises that the vast majority of service personnel make this adjustment successfully and are not more likely to commit a violent offence post-deployment - there is only an increased risk of 2% when compared to the general population.
"However, any violent offence is unacceptable and will not be tolerated by our armed forces."
The Royal British Legion said: "The vast majority of ex-service personnel go on to live successful and law-abiding lives. However, inevitably, and for a variety of reasons, a small number experience difficulties."