As the U.S. is reorganizing its remaining forces in South Korea, to make it easier to move American forces into, and out of, the country, it is also preparing for war with North Korea. To this end, the main army headquarters, for the Eighth Army, is being upgraded with staff and support personnel so that it could quickly command twelve or more combat brigades, and all the support forces needed for that many combat troops. For the last decade, the U.S. has been working to transfer supreme military command in South Korea to the South Koreans. However, as North Korea developed nuclear weapons, and built more long range ballistic missiles (and threatened to use chemical weapons), South Korea sought more potential help from the U.S., if North Korea should attack. So while U.S. troop levels in South Korea are at record lows, preparations are being made to increase them, a lot, in a short period of time.
The American troops have been in South Korea for over sixty years, 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 70s there were only 42,000. There it stayed for over two decades. Then came the 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 in 2008 it went to 28,000.
In the last few years, the U.S. has been moving its forces away from the DMZ (Demilitarized Zone) that forms the border between North and South Korea. This makes it possible to let troops bring their families with them. That means troops will stay for three years when assigned to South Korea, instead of the previous 12 month unaccompanied (by family) tour of duty. By keeping troops around for three years, they get to know about South Korea and their counterparts in the South Korean military.
The army may even, like the air force, bring combat units in for training exercises, then send them home. This will test Eighth Army capabilities to receive reinforcements and quickly get them into action. Until the government in North Korea collapses, or undergoes extensive reform, Korea remains a place where the United States could be involved in another war.