The moral complexity of the Vietnam war made it a war like no other, to our experience at that time. The combination of the foreign and complex cultural issues that resulted in the war and the draft made serving in Vietnam very different from every war before and since. Since soldiers did not choose to fight in the war, they were lost in a moral quagmire. My own father fought in Vietnam, from the draft, in his late teens. The war both established and twisted his moral compass. Rules of Engagement aside, to be a solider in Vietnam was to fight with little support in a frightening, dangerous and foreign land. To say that most of them were miserable is an understatement. For some, who perhaps already had those tendencies, sociopathic or psychopathic behavior resulted. This was a result of who was in the war (young men who either did not want to be there or wanted to go for some reason) and the moral never-never-land they found themselves in.
I have read numerous accounts of frustration with the ROE particularly as they applied to the air war. Fighter pilots chasing migs and having to break off because they went over restricted air space or the frustration of both fighter and bomber pilots at having to back off of their bombing schedules after having eliminated much of the SAM defense systems of the NVA. This left them time to re-arm and re-build and then shoot down more US pilots as opposed to keeping the pressure on and not allowing them to regroup.
I think lrwilliams makes a good point. Having no personal experience with a Vietnam Veteran, I cannot express my ideas with 100% certainty. However, I could make a pretty educated guess, that the ROE caused problems for the soldiers because of unclear objectives from the United States government. My understanding is that the objectives, if there were any specifically stated, were ambiguous. That would have then caused a problem for the ROE in that soldiers were told to do one thing and they were limited in how this was to be accomplished.
This highlights one of many reasons the Vietnam "Conflict" was so frustrating to the soldiers involved and so misunderstood by the people back home. It is my opinion that if you are going to go to war you need to have the intention of being on the offensive at some point and attempting to "win" the war.
As the previous posts have mentioned, American soldiers understood the rules of engagement during Vietnam (even if they didn't understand the motivation behind those rules). The difficulty came when troops were placed in precarious positions and knew that they could not "fire unless fired upon." My dad was in Vietnam from 1969-1970, and he said that the ROE were one of the most frustrating elements of his combat experience. He felt like a sitting duck and questioned what in the world he was doing there if he was simply to remain on the defensive. This was particularly discouraging to soldiers like him who voluntarily went to Vietnam (one of the myths about the conflict is that most U.S. forces were drafted for it--actually, two-thirds volunteered, more than the percentage of volunteers for WWII).
Similarly, my husband said that when he was in Iraq that the ROE were even stricter (during his second deployment). If someone came toward him with a weapon, my husband had to prove that that person was a threat.
A variety of accounts like the one highlighted in #3 above, have developed the frustrations and difficulties faced by the average soldier in Vietnam thanks to the rules of engagement that were imposed upon them. It does not take much ability to empathise to see how some of the situations facing soldiers, such as needing a legitimate provocation before opening fire, could have been incredibly taxing, mentally and psychologically.
The Rules of Engagement got increasingly restrictive as the war went on, and politicians and military leaders began to try and "Vietnamize" the war by putting American troops in less offensive operations. From 1970 on, the Vietcong and North Vietnamese Army, with notable exceptions, were mostly content to simply wait us out.
One good example of the frustration experienced by American soldiers is described in the novel CW2, by Layne Heath. Heath, who spent three tours in country flying Huey helicopters and Kiowa scouts, details how towards the end of the war they were not allowed to shoot unless fired upon. He would deliberately make his aircraft vulnerable to ground fire so they could report being shot at and counterattack. To him it seemed an unnecessary risk for troops who were clearly at war.
He also discusses that in some cases they were forced to call in for approval to fire on suspected Vietcong, then watch as they escaped while waiting for approval from base.
As the post above says, it was very much like fighting with one hand behind your back, and had to be the ultimate in frustration for a soldier.
To my knowledge, there was not a problem with individual soldiers understanding what the rules of engagement were in Vietnam. The problem for soldiers in Vietnam was accepting these rules. The restrictions placed on the military by the rules of engagement tended to be one of the factors that lowered morale in the military.
The rules of engagement lowered morale because they seemed to force the military to (at times) fight with one hand tied behind its back. This seemed to individual soldiers as if they were being asked to risk their lives more than necessary (because they couldn't fight back freely) simply for the sake of some rules imposed for political reasons.