The US has withdrawn most of its troops from the truce village of Panmunjom on the North-South Korean border.
Panmunjom hosted 220 US troops
The move is in line with a general reorganisation and reduction of US troops in South Korea. The Pentagon says better technology will compensate.
The cut in US troops on the border will be made up by South Korean forces.
Washington is planning to pull a third of its troops out of South Korea altogether by 2008 as part of a global realignment plan.
"As previously agreed, transferring the role of security work at Panmunjom has been completed," a spokesman for the US military command told South Korea's Yonhap news agency.
Panmunjom, in the middle of the 4km (2.4 mile) demilitarized zone (DMZ) separating North and South Korea, is where the armistice which ended the 1950-53 Korean War was signed.
The US cut its troops from 220 to about 40. South Korean forces will make up the rest of the 550-strong contingent.
Although the Pentagon has stressed that advances in longer-range missiles and technology will make up for the reduction and pulling back of its troops, the changes have made the South Korean public nervous.
Seoul is only 50 km from the North Korean border.
As a result, the US recently made the compromise of phasing its withdrawal of nationwide troops - initially to be completed by the end of 2005.
The plan is that 5,000 US troops will leave South Korea this year, 3,000 next year, 2,000 in 2006, and 2,500 in 2007 and 2008.
That will leave a total of about 24,500 troops in the country.
The US and South Korea also agreed earlier this year to relocate all of the US troops based in the South Korean capital, Seoul, to a new base further south.
The US has said it will move the 8,000 troops to Pyongtaek, 80km (50 miles) to the south, by December 2008, freeing up money from its prime real estate location.
Both sides have also agreed to eventually relocate 14,000 troops currently based between Seoul and the North Korean border.
Their forward position puts them in range of North Korean artillery, and US officials have said that pulling troops back south of the capital would strengthen the military's hand.
US troops have been stationed in South Korea since the Korean War.