Soldiers suffering psychological traumas years after serving in a war also experience poorer physical health, a series of studies suggest.
Psychological trauma is linked to poorer physical health
Those with post-traumatic stress disorder were more likely to develop heart disease and cancer in later life than fellow war veterans, they showed.
The reasons are unclear but may be down to stress hormone levels, experts say.
New Scientist pieced together evidence including surveys of more than 18,000 who had served in the Vietnam war.
Latest research, to be published in Annals of Epidemiology, re-analysed data gathered originally by the US Centers for Disease Control and Prevention in 1985.
Dr Joseph Boscarino and his team at the New York Academy of Medicine divided the 18,000 veterans into those who suffered from post-traumatic stress disorder (PTSD) and those who did not.
They found stark differences in death rates between the two groups over 30 years after the Vietnam war had ended.
The men with PTSD, whether because of their combat service or not, were far more likely to die from accidents, drugs or suicide.
However, those who developed PTSD as a result of the stresses of war were also more likely to die of heart disease and various types of cancer.
Dr Boscarino said: "The excess deaths in both PTSD groups show that stress can kill. But the much greater effects among combat veterans shows there is something especially bad about that."
A recent report in Social Science and Medicine showed Israeli veterans with PTSD after the combat in Lebanon in 1982 were twice as likely to have high blood pressure, ulcers and diabetes and five times as likely to have heart disease and headaches as those who did not develop PTSD.
The authors, including Yael Benyamini from Tel Aviv University, said PTSD was the key mechanism that leads from the trauma to poorer health.
However, it is not clear how.
Dr Boscarino said the increased mortality found could be related to biological, psychological or behavioural factors linked with PTSD.
Research suggests anxiety and depression can have a negative effect on the body, such as making the heart more vulnerable to irregular heart beats and increase the risk of blood clotting.
Stressed war veterans may also be more likely to lead unhealthy lifestyles, smoking and drinking excess alcohol, which could in turn lead to poorer physical health.
In Dr Boscarino's study, differences in smoking habit did not explain the difference in cancer death rates between those with and without PTSD.
Shaun Rusling, vice-chairman of the National Gulf War Veteran and Families Benevolent Association, said exposure to biological agents during warfare might also be a factor.
"What is clear is that war is not good for you. That is a fact," he said.
"Servicemen who have been in combat zones should have extra health screening throughout life," he said, but added that this was not happening at present.