Unlike previous conflicts, individual personnel 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?
The 12 month tour of duty (along with a 6 month tour in troop command for officers) is seen as one of the worst policies of the Vietnam Era. It is seen as something that made unit cohesion, which is extremely important for morale and fighting effectiveness, very difficult to achieve.
In wars like World War II, military personnel had been committed to the military for the duration of the war. This meant that they would often stay with their units indefinitely. This led to strong unit cohesion as men stayed together for long periods of time and shared common experiences. The policy in Vietnam made it so that this could not happen.
The Vietnam policy led to major rifts. It led to feeling of extreme isolation for new personnel rotating in, who were looked down upon by their more veteran comrades. It led to a desire to simply stay alive for those who were approaching the date on which they were to transfer out. This meant that they would be less likely to take risks for the good of the unit or their fellow soldiers.
This policy was believed to be necessary because so many of the soldiers were draftees who were only in for two years. The need for training and other factors meant that a soldier in for two years could only realistically be committed to Vietnam for one year.
While the policy was deemed necessary, it is now seen as a major blow to the morale and unit cohesion of the American military in Vietnam.