South Korea is decreasing the amount of money it pays towards the costs of American troops being stationed in South Korea. Previously it contributed $790 million a year, but starting this year, that will be reduced to $692 million. There are currently about 28,000 American troops there, and it costs the United States nearly $4 billion a year to maintain that force in South Korea.
American troops have been in South Korea for over 60 years, since the end of World War II in 1945. 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 the September 11, 2001 and the war on terror. U.S. troops were needed elsewhere. By 2004 the U.S. force in South Korea was down to 37,000. In 2006 that dropped to 30,000 and this year will go to 28,000.
The troops are there to help protect South Korea from any other attack by North Korea (whose 1950 invasion led to a three year war, and a 56 year old ceasefire.) Babysitting a cranky North Korea has been seen in American's interests. As South Korea's economy has boomed, payments to defray the expense of U.S. troops was begun, in 1991, and have been increasing. The 1991 payment was $150 million.
Such payments were first seen in Germany and Japan, where the U.S. urged those nations to help cover the cost of having American troops there to help defend them during the Cold War. Those payments continue, if only because the U.S. forces are also a major contributor to the local economy.