Unlike previous conflicts, individual personnel in the Vietnam War transferred into units that were continually deployed and transferred out individually, as well as on their own rotation dates. What dynamic did this introduce to unit cohesion, and to the individuals as they transferred?
This question makes the most sense in the context of the Vietnam War so I have edited it to refer to that conflict specifically.
There is some disagreement on this particular issue. For example, the article in this link denies that there were any negative consequences brought about by the system of personnel rotation in this war. However, the conventional wisdom is that the way that troops were rotated in and out of Vietnam contributed to a breakdown in unit cohesion. A statement of the conventional wisdom can be found in this link.
The conventional wisdom holds that the US Army harmed itself through its policy of personnel rotation. Because personnel rotated in and out as individuals and not as units, soldiers did not experience the sort of camaraderie and unit cohesion that would have been present had they moved in and out of combat at the same time. Because the Army moved officers in and out of combat units, those units were constantly having to adjust to new leadership in the stress of combat. This was arguably very bad for morale. Soldiers in the various units did not come to have a sense of all being in the war together. Instead, they felt isolated from one another and were more concerned with their individual situations than with their unit as a whole.