By Jonathan Marcus BBC defence correspondent

Large numbers of American troops are to be pulled out from long-standing bases in places like South Korea and Germany.

Many will return home, ready to be rapidly deployed to new bases in places like Central Asia.

The war in Iraq has to some extent influenced the timing of US plans. US forces are increasingly over-stretched - another factor prompting the reduction in permanent overseas commitments.

US TROOPS IN SOUTH KOREA US to pull out 12,500 troops Significance largely symbolic: S Korea has 690,000 troops, N Korea 1.1m US plans to pull remaining troops back from DMZ

A US brigade that is being withdrawn from Korea will be heading to Iraq.

But where does all this leave South Korea's defences?

The scaling down of US forces should accelerate a trend that has been underway for a number of years, with the South Koreans taking more responsibility for their own defences.

South Korea has relatively modern and effective forces.

While some US units are to be pulled out, those that remain are being modernised, and many of them are to be relocated south of the capital Seoul, further away from forward deployed North Korean forces.

It is all part of an unprecedented period of change in the US-South Korean alliance.

While it might make strategic sense, there are concerns about the political signals this redeployment could send.

Some analysts fear that North Korea will see the US troop reductions as a sign of weakness.

And many observers stress that the re-balancing of the defence relationship between Seoul and Washington will inevitably have an impact on the wider relationship between the two countries.