The United States is withdrawing a mechanized brigade (the 1st Brigade of the 2nd Infantry Division) that has been in South Korea since 1965. The 4,600 personnel in the brigade will be replaced by brigades that will be rotated in and out regularly. Like the current “permanent” brigade the equipment and weapons will always stay in South Korea. But unlike the past, where individual troops arrived in South Korea to replace those in the brigade whose 12 month tour was up, the new system will replace all the personnel in the brigade at once. The advantage of this approach is that it allows brigades to train for up to a year in the United States and then go over to South Korea, combat ready and fully trained as a unit for up to a year of duty as a combat ready brigade in South Korea. This approach was used extensively and quite extensively since 2001 in Afghanistan and Iraq.
This comes after decades of effort by a determined group of officers to get the army to accept some lessons learned during World War II. The most important lesson here is that small units of troops must be kept together, and replacements for casualties integrated into the unit carefully. This has been done since the 1990s and the capabilities of the combat units have been astounding. But many journalists and politicians are still unaware of how important this "cohesion" thing is.
Cohesion is nothing more than keeping small units of combat troops together long enough for them to bond, and become an effective team and then to train as a team. Same things as with sports teams. But from the end of World War II, until the late 1990s, the "individual replacement" system was used, which constantly destroyed this cohesion. Combat losses were replaced on an individual basis. Same thing in peacetime. When a soldier left, usually at the end of his enlistment or tour of duty, a single replacement was brought in. This meant that units lost over five percent of their troops each month. Where this hurts was 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 its combat edge back. Same with a tank or artillery crew. Or even a team of clerks or mechanics.
The alternative was 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 appreciate waiting until they are back home to incorporate new people. The battlefield is not the place to do that.
The U.S. Army has, since the 1990s, been making more of an effort to send entire units overseas, instead of individual replacements. This is the "cohort system" and the army is extending its use from combat, to combat support units. It's not been an easy transition.
Now the army is using the cohort system for combat support units in non-combat assignments. Patriot anti-aircraft missile battalions are now rotated overseas 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 overseas, with just the troops moving.
What's amazing about this is that the problem was identified and analyzed at the end of World War II. Academics (who were often combat veterans) wrote papers about the phenomenon. But many senior generals didn't get it. It wasn't until after Vietnam that the senior brass recognized the problem enough to start implementing a solution. Naturally, the "cohort" system costs more money. In the Pentagon there are many special interests looking for that money. For the moment those special interests aren't grabbing some extra cash at the expense of stability and capabilities of the combat units.
All this is now being applied to the few American Army troops left in South Korea. U.S. troop strength in South Korea continues to shrink, because South Korea now has better trained, led, and equipped forces than in the past. The American troops have been around for over half a century, and the U.S. has always said it would stand by its South Korean ally. But the numbers tell a different tale. At the end of the Korean War, in 1953, there were over 350,000 U.S. troops in South Korea. Within a year that shrank to 223,000, and by 1955, it was only 85,000. By the mid-60s, it was 63,000. By the mid 70's, there were only 42,000. There it stayed for over two decades. Then came September 11, 2001, and the war on terror. By 2004, the U.S. force in South Korea was down to 37,000. In 2006, that dropped to 30,000 and it is now 28,000. There is fear that the U.S. will cut the American force in South Korea to token (a few thousand troops) size. Meanwhile, more Americans are getting quite vocal about why there should be any U.S. troops in South Korea at all. Enough is enough, and over half a century of paying to supply South Korea with a protective garrison should come to an end.
While North Korea has long maintained a large (nearly a million personnel) military, these troops are poorly led and equipped and there has been little cash for new equipment or training since the 1990s. In the last two decades the South Koreans have upgraded their own military to the point where it is considered on par with U.S. troops. But decades of threats from North Korea have instilled a degree of fear in South Koreans that cannot be shaken. The farther you are from Korea the more absurd the North Korean threats appear to be. But if you live within range of North Korea rockets or artillery, it’s hard to get a good laugh out of the situation.