To stand at Panmunjom - the last border of the cold war - is to feel North Korean hostility. Armed guards fix their binoculars on visitors on the southern side, even though they might be standing just metres away. Behind them, hidden in the barren mountains, is a military that makes the border between North and South Korea the most fortified in the world. Much of North Korea's 1.1 million standing army - one of the biggest in the world - is backed by an estimated 4,000 tanks and, dangerously, a complex network of artillery pieces hidden in tunnels kept on a high state of alert just beyond the border. Seoul - one the region's most modern capitals - is less than 100km away. As the region grapples for a meaningful diplomatic response to North Korea's latest nuclear test on Monday, the military realities on the ground reveal just how important that search is. A military response, even a so-called surgical strike to take out suspected nuclear targets, is considered virtually unthinkable by veteran regional envoys and intelligence analysts. Israeli jet fighters bombed a suspected nuclear facility in Syria in September 2007 - an act that would be far more risky to attempt against a country as prepared as North Korea. Even, for the sake of argument, if potential hurdles such as winning Chinese and Russian backing for such action were removed, and North Korea's short- and medium-range missiles ignored, Pyongyang has enough conventional weaponry trained on Seoul to make any such move extremely dangerous. There is little doubt it would be perceived by a regime as insular and paranoid as that in Pyongyang as an act of war. 'You only have to look at the border to see that North Korea is not a normal country - the paranoia has bred an extreme military build-up,' one Asian defence attache said. 'There are huge air defences, for starters. And there is no one better at exploiting tunnels and bunkers. Just knowing what they've got and where they've got it - even the basic stuff - is fraught with risk. 'We are talking about a country that has been obsessively preparing for just these kinds of situations - shutting down the options of its enemies.' In fact, the fear of invasion is so great, it is widely considered to be one of the factors fuelling Kim Jong-il's drive for a successful nuclear weapons programme. The US has never engaged in a conflict with a nuclear-armed state. As the search intensifies for a workable diplomatic solution to punishing Pyongyang and/or getting it back to the six-nation talks on disarmament, the options seem disturbingly narrow. But not all of the news is necessarily bad. Nuclear scientists still absorbing seismic data to gauge the strength of Monday's test - detonated in a hole 6km beneath a remote mountainside in the northeast - believe it may have been smaller than first reported. While initial Russian estimates pointed to a blast of Hiroshima proportions, several independent estimates suggested it was something considerably smaller - which probably points to technical problems. This means the effort to perfect the shrinking of a nuclear device to fit on a warhead that could be placed on one of its long-range missiles - Washington's biggest fear - could still be some way off. 'My guess is that North Korea tried and failed to get a simple plutonium bomb to detonate correctly,' Jeffrey Park, director of the Yale Institute of Biospheric Studies, said in the Bulletin of the Atomic Scientists. 'Make no mistake, an inefficient nuclear weapon is nothing to dismiss. Even at the low end of its estimated yield [2 kilotonnes], the May 25 test released as much - or more - explosive energy than the largest conventional-explosive air raids during World War II,' said Professor Park. 'But one should be mindful of the technical challenges North Korea still faces in carrying out the threats implied by its deliberate pairing of its explosive test with test missile launches.'
