South Korea has dropped its reference to North Korea as its "main enemy", in the country's latest White Paper on defence.
North and South Korea are still technically at war
Seoul began officially labelling its northern neighbour enemy territory in 1995 but the last time it did so, in 2000, North Korea strongly objected.
This year, Seoul simply lists the North as one of several "military threats".
The report comes as momentum builds for multi-national talks on Pyongyang's nuclear programme.
The South Korean defence ministry said in a press release that it had decided to change its reference to the North in recognition of its complicated ties with its neighbour.
Economic ties are improving, but the political situation remains extremely sensitive.
"We considered the extraordinary and dual nature of inter-Korean ties - that we have to push for inter-Korean rapprochement and military preparedness simultaneously," the ministry said.
The 2005 White Paper noted, however, that North Korea's military arsenal is increasing.
The North has added 1,000 pieces of military equipment - artillery guns and multiple rocket launchers - to its 1.2m-strong army, the paper said.
The report said that, in the event that war broke out on the Korean peninsula, the US would sent 690,000 troops and several boats and aircraft.
"The United States has a plan to send more than 40% of its entire navy, more than half of its air force and more than 70% of its marine corps to defend South Korea," the White Paper said.
Seoul and Washington forged their alliance during the 1950s, when US troops led UN forces to help South Korea against the North.
The war ended in a truce, not a peace treaty, leaving the peninsula still technically at war. In recent months, the Americans have cut back their troop presence in South Korea.
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 US, Seoul and other Asian nations are all concerned to help mitigate any effects of a war in the region by persuading Pyongyang to relinquish its nuclear weapons programme.
Talks on this have been stalled for several months, but North Korea has recently said it is ready to begin negotiations again soon.
A Chinese envoy will visit the North later this month to try and jump-start them, a South Korean official said on Friday.