These days, however, the welcome has cooled for the GIs who fought alongside South Koreans and troops from 16 other nations to drive communist forces back to North Korea during the 1950-53 Korean War.

Over the years, tension has never been far below the surface on the Korean peninsula, divided by the world's most heavily fortified border across which massed armies still stand ready to fight at a moment's notice.

Many South Koreans agree that the US military presence has played a crucial role for South Korea's security and economy against North Korea's threatening military posture. They remain thankful that some 36,000 young Americans gave their lives during the conflict that ended 50 years ago on Sunday with the signing of an armistice agreement.

"Over the last half century, US soldiers have successfully carried out their duty to prevent war on the Korean peninsula," said Baek Seung-Joo of the state-run Korea Institute of Defense Analysis.

Now, however, the United States finds itself in an awkward position, with a brasher and more assertive generation of Koreans ready to question the blind loyalty to Washington of many of their parents.

Cultural and political differences often make it difficult for US troops stationed here under a mutual defense pact to understand why anti-US sentiment is growing in South Korea.

South Koreans still regard US ground forces as a "trip wire" along the world's most heavily armed frontier, ensuring that an invasion from North Korea would immediately draw the United States into conflict. For them US troops are needed as much now than they were half a century ago.

However, analysts say South Korea, strengthened by a growing sense of security, has grown up enough to demand it be treated as a sovereign security partner, rather than a mere client state of the United States.

Relations soured further after the death last year of two schoolgirls crushed by a US military vehicle that sparked widespread anti-US protests in a population that has a love-hate relationship with the United States.

Anti-US demonstrations have subsided this year, with the spotlight focusing on North Korea's nuclear ambitions.

"The collapse of the Soviet block has prompted a change in military ties between Seoul and Washington," said Kim Il-Young, a political science professor at Seoul's Sunkyunkwan University.

The United States appears to be realigning its forces in South Korea and Japan so that they can serve as regional power balance, rather than a deterent to war on the peninsula, he said.

The emergence of China as a world power is a key factor in US strategic thinking, according to Kim.

"China appears to be the target of current US troops realignments," Kim said.

The United States mapped out a long-term scenario in 1990 to redeploy its forces in East Asia. The plan, however, was shelved in 1993 when North Korea's nuclear brinkmanship roiled the Korean peninsula.

Tension escalated again after North Korea allegedly admitted in October last year that it had launched a secret nuclear weapons program.

The traditional US-South Korea alliance was based on the perception that the Korean peninsula is strategically vital.

"The perception, however, has been changing, with US confidence in its firepower gaining after the Iraq war," Kim said.

"If the nuclear crisis subsides, the United States will push ahead with its plan to transform US forces in South Korea into a task force covering the entire region," he added.

Korean War veterans from more than 16 nations will hold a historic ceremony in the truce village of Panmonjom Sunday marking the signing of an armistice that ended the conflict 50 years ago.

The armistice was a key legal document that has kept an uneasy peace on the divided Korean peninsula.

But South Koreans see no call to celebrate the anniversary.

"With the armistice seen as a dead document for North Korea, the presence of US troops is still vital to our security," Baek said.

WAR.WIRE