from Strategy Page
...the U.S. forces in South Korea have shrunk from over 100,000 troops in the early 50s (after the war ended), to under 30,000. These days, the well equipped South Korea forces are believed capable of handling any invasion from the north.
At the same time, communist North Korea has suffered famine and economic collapse since the end of the Cold War in 1991, and the end of Russian and Chinese subsidies that propped up the mismanaged economy. The North Korean military has, especially in the last decade, declined because of lack fuel shortages, which limited training. There hasn't been much money for new equipment, either, and the current stuff is falling apart.
The North Koreans are still a threat, but South Korea is more worried about the human and fiscal fallout from a collapse of the North Korean government, and a reunification of Korea. That chaos will be paid for by the newly affluent taxpayers of South Korea, and the policed initially by South Korean troops. The small American force will, as always, be there mainly to guarantee U.S. reinforcements if the Chinese march into South Korea via North Korea, or the North Korea come across the border and get lucky.