Summaries of representative studies are below:

Hoge, C. W., Castro, C. A., Messer, S. C., McGurk, D., Cotting, D. I., & Koffman, R. L. (2004). Combat duty in Iraq and Afghanistan, mental health problems, and barriers to care. New England Journal of Medicine, 351(1), 13-22.

A study conducted among the US Army and Marine corps personnel who served in Iraq and Afghanistan found that there was a strong relationship between combat experiences (being shot at, handling dead bodies, knowing some who was killed, killing enemy combatants) and the prevalence of PTSD.  (Hoge, 2004)

Seelig, A. D., Jacobson, I. G., Smith, B., Hooper, T. I., Gackstetter, G. D., Ryan, M. A., ... & Smith, T. C. (2012). Prospective evaluation of mental health and deployment experience among women in the US military. American journal of epidemiology, 176(2), 135-145.

A similar large scale study, the Millennium cohort found that women who were deployed and reported combat-related exposures had greater odds of a psychiatric condition (PTSD, depression and anxiety) compared to those women non-deployed. (Seeling, 2012)

Betancourt, T. S., Newnham, E. A., McBain, R., & Brennan, R. T. (2013). Post-traumatic stress symptoms among former child soldiers in Sierra Leone: follow-up study. The British Journal of Psychiatry, 203(3), 196-202.

A study in war affected Sierra Leone found that among child soldiers (average age 16.6), 36% endorsed having injured or killing someone and 32% met the criteria for PTSD.  War experiences were a significant predictor of increased PTSD. (Betancourt, 2013)

Jacobson, I. G., Ryan, M. A., Hooper, T. I., Smith, T. C., Amoroso, P. J., Boyko, E. J., ... & Bell, N. S. (2008). Alcohol use and alcohol-related problems before and after military combat deployment. Jama, 300(6), 663-675.

A fourth study, found among Reserve/Guard personnel who deployed with combat exposures were also observed to be at increased odds for new-onset of alcohol-related problems including: heavy weekly drinking and binge drinking (Jacobson, 2008)