US-South Korea military alliance under pressure
The latest tensions on the Korean peninsula are, among other things, a test for the US military alliance with South Korea.
It is a very different world from the one in which the alliance was forged.
For Washington and Seoul, the challenge is juggling what they would see as moves to maintain the credibility of the alliance, reassure opinion in South Korea and deter the North.
Pyongyang protests such moves are a provocation, against the backdrop of other important regional neighbours - not least China - looking on with their own sensitivities and concerns.
This last week's flare-up is considered one of the most serious since the end of the Korean War. And it has come just months after the sinking of a South Korean warship with the loss of 46 South Korean sailors, which was widely blamed on the North.
Indeed, the blame game between North and South adds to the uncertainties. But it has prompted questions in Seoul and Washington about what more might need to happen in terms of escalation to prompt a tougher response from either the South Koreans or even the US.
Given that there is no appetite in Washington - or anywhere else really - for a full-scale military confrontation, the judgements are especially acute.Force reduction
The US-South Korea military alliance stems from 1954, and the aftermath of the Korean War.
Under a mutual defence treaty, the US agreed to help South Korea defend itself, especially against an attack from the North. Washington has maintained a continuous military presence in South Korea ever since.
The ebb and flow of tensions on the Korean peninsula has also affected the relationship between Washington and Seoul.
South Korea has also developed substantially both economically and militarily, and is more able and willing to take on greater security responsibilities. But, as the recent tensions have highlighted, that still applies perhaps only up to a point.
The alliance was strained earlier in the decade. The attitudes of Washington and Seoul towards North Korea seemed out of step.
The Bush administration was adopting a hard line on the North's suspected nuclear ambitions, while the then government in Seoul was pursuing its so-called "sunshine" policy of developing contacts with Pyongyang.
The US military presence was also unpopular with South Korean opinion at the time, fuelled in particular by the deaths of two South Korean schoolgirls in 2002 in a road accident involving a US military vehicle.
Because of this, US preoccupations elsewhere and a push by the Pentagon to cut its military force levels in various parts of the world, there have been significant changes to the US military presence.
Personnel numbers have been reduced from 37,000 to 28,000. But there have been delays in plans to move US forces further south from the bases that they have been occupying.
The plan to change the operational command of South Korean troops in wartime has also been put off. Currently, they would come under the command of US forces if war broke out, whereas the plan is for separate commands in the future.
All these various changes and plans have led some critics in Seoul and Washington to argue that the deterrent effect of the US-South Korean alliance has been weakened.
Clearly, it is a time when strategic relationships throughout the region are in flux, and when the leadership in Pyongyang seems even more unpredictable than usual.