During World War II, most men joined the service out of a sense of duty and a sense of right and wrong. During the Vietnam war, many men, but not all, were drafted and really didn't want to be there. But they didn't want to go to prison. But men tend to join today's volunteer army so they'll have a job. These are all generalizations of course, but only for the purpose of discussion.
Ethics is a deep and complex subject. Right and wrong may seem easy to determine on most occasions, but what's not easy, at all, is to give reasonable and rational arguments for what's right and wrong on all occasions. That's why ethics is an imprecise branch of philosophy that often asks more questions than it can ever hope to answer. Some moral dilemmas are impossible to resolve.
If a man goes to war and kills out of a sense of duty, then that sense of duty may protect him from some of the psychological consequences of committing violence. He can tell himself that his heart was in the right place and his intentions were good—-and they probably were. But in the heat of battle, things happen.
If a man goes to war because he feels he was forced, that may also protect him from some of the psychological consequences of committing violence. But probably not as much as the man who went to war out of a sense of duty. He may experience doubt. “Why didn't I resist? Why did I follow orders I didn't agree with? If I went to war because I was afraid of going to prison, does that absolve me of responsibility for what I did? Maybe I did it just to save my ass. And that boils down to self-interest.”
But the man who volunteered so he would have a job is likely to have more demons to wrestle with than the other two men. Again it's a generalization, but he joined the army for a paycheck. In a sense that makes him a mercenary soldier. Is this an unfair generalization? Of course it is. But when a soldier starts regretting the acts of violence he committed for his country, his conscience is not likely to play fair with him. And he may have even less psychological refuge from the horrors he participated in than the other two men.
Maybe there are psychological consequences to an all volunteer army that haven't been thoroughly considered. And if those men are at a higher risk for post traumatic stress disorder because of that, then the military has a duty to take that into consideration and help these men to the best of their ability.
Of course these three ways of looking at the morality of the decision to go to war don't even begin to ask all the pertinent moral questions that could be reasonably raised. But war is perhaps the ultimate moral dilemma and these questions are a good starting place, because veterans of wars are likely to ask these questions of themselves even if no one else asks them. And their answers—-whether they are the right ones or not—-might torture them for the rest of their lives. Ultimately they may not be able to resolve their moral dilemmas, but they must face them and come to terms with them. This is where ethical questions meet psychological considerations. The mind naturally debates itself about these matters. And coming to terms with this internal debate, for these veterans, may make the difference between a lifetime of hell or a second chance at life.
Ethics is a fascinating branch of philosophy. And it's arguably the most important branch. For anyone who's looking for a quick, broad, detailed, and painless introduction to ethics, there's no book that even compares to “The Ethics Toolkit,” by Baggini and Fosl.