From the end of World War II, until the late 1990s, the "individual replacement" system was used. Combat losses were replaced on an individual basis. Same thing in peacetime. When a soldier leaves, usually at the end of his enlistment or tour of duty, a single replacement is brought in. This means that units lose over five percent of their troops each month. Where this hurts is at the lowest level. An infantry fire team, of four or five troops, is only as effective as it is coordinated. Take one guy out and replace him with a new soldier, and it takes weeks, or months, for that team to get it's combat edge back. Same with a tank or artillery crew. Or even a team of clerks or mechanics.

 

The alternative is to form units, keep them together through training, then send them overseas, or hold them ready for an emergency, for about a year. While some troops are lost to normal attrition (illness, disciplinary problems, or combat casualties), the unit is largely intact. This approach has worked wonders on the battlefield. Troops know the people they are working with, and and appreciate waiting until they are back home to incorporate new people. The battlefield is not the place to do that.

 

Now the army is using the cohort system for combat support units in non-combat assignments. Patriot anti-aircraft missile battalions will now be rotated to South Korea as complete battalions. Formerly, individual batteries had been sent over, and these functioned more effectively than batteries that used individual replacements. It follows that the entire battalion will be more effective if all the troops are sent over at once. All the weapons and equipment for the battalion stay in South Korea, with just the troops moving.