One of the worst injuries ever inflicted on American infantry was the practice of treating units as a collection of individuals, rather than a group of soldiers who obtained their combat effectiveness from training and fighting together for a long time. During World War II, the "individual replacement" system was introduced. While other armies, as they had done for centuries, brought in replacement troops when the unit was not in combat, and then trained the new men to work well with the veterans, the new American system sent individual troops to units in combat. Soldiers were treated like replacement parts for a machine (the unit that needed replacements for dead or wounded troops.) The policy was a disaster, but has resisted repeated attempts to eliminate it. In practice, the new replacements are shunned by the combat veterans. The new guys are inexperienced and, in combat, inexperience and unfamiliarity tend to be fatal. The new replacements often did not last the night, as they were often told to "go dig a foxhole over there and don't do anything stupid."
Many World War II divisional commanders, who had some control over what was done with the infantry replacements, put the new soldiers through additional training and then moved them into the infantry companies that needed them after the company had been pulled out of combat for at least a few days, if not longer. The veteran infantrymen in these companies appreciated this more reasonable policy, and would often work hard with the new guys to get them ready, and get to know what their strengths and weaknesses were. In divisions like this, the losses for replacement troops was much lower.
While many generals knew the individual replacement system was a colossal failure, after World War II the army adopted the policy of carefully managing each soldiers career. This was done because it was seen as a modern way to manage people, and, at least in peacetime, an attractive feature that would keep officers and troops in the army. There was some wisdom to that angle, but every time a war came along (Korea, Vietnam), the combat units continued to get individual replacements while they were in action, or about to go into action. World War II vets in Korea and Vietnam winced at seeing the same high attrition among new replacements brought into units during combat operations. Again, local commanders improvised to see that the replacements were allowed to train with their new unit before entering combat. But this was compromised by the "one year combat tour" system introduced during the Korean war. This idiotic policy came about because so many World War II vets, convinced that joining the reserves would only get them back into combat if World War III broke out, found themselves sent to an unexpected war in Korea. The one year tour was a political solution to voters unhappy with their husbands and sons being sent off to another war. But this cause constant turnover in combat units. More individual replacements had to be brought in to replace men who had finished their tour, as well as those who were killed or wounded.
The "cohort manning" approach is supposed to keep officers and troops in the same combat unit for two or three years. This won't be easy, as there are so many schools and temporary assignments available to take men out of their unit. It's no secret that the longer troops are together the better they perform as a unit. This has been seen for years as reserve and National Guard units, which only train on weekends and two weeks during the Summer, constantly beat regulars at tasks that require team work. Israel has the same experience with it's reserve units, which are more frequently put to the test in combat operations.
It's a strange situation. All the combat veteran generals have known, and experienced, how the individual replacement system fails. But for sixty years, no one has been able to do anything about it.
The army is going to try, as it has about once a decade over the last half century, to keep troops in combat units long enough so that they can become effective. A "cohort manning" will be established so that officers and troops don't tend to get transferred out of their units just as they have learned how to be a valuable member of a combat team. This effort will probably fail, like all other attempts to eliminate the "individual replacement" system have.