By John Sudworth
BBC News, Seoul
Tourists and workers had become used to a more open border
The journey from Seoul to Kaesong is just a short drive between two cities on the same peninsula.
But the special tourist buses, which began running 12 months ago, are laden as much with symbolism as they are with passengers.
For thousands of South Koreans the chance to visit a real North Korean city, and to glimpse the reality of daily life, is an act of pilgrimage and time travel rolled into one.
At least it was.
From 1 December, the scenes of oxen-ploughed fields and Soviet-style cityscapes are once again out of bounds, hidden beyond one of the world's most impermeable borders.
That they were ever visible at all is remarkable.
From the end of the Korean war, for almost half a century, North-South relations were stuck in a seemingly unbreakable deadlock of hostility and mutual loathing.
Then in 1998 things began to change.
For the next decade, two successive liberal governments oversaw a "sunshine policy" of engaging, rather than isolating, the old enemy.
Billions of dollars in aid and investment were showered on North Korea, a country whose command economy was in tatters, and whose people had just suffered a devastating famine.
The Kaesong tourism project was only the latest development in what some commentators believed was to be a gradual thawing of hostilities between the two Koreas, and the slow opening up of the North.
Lee Myung-bak has insisted the North get rid of its nuclear bomb
But in February this year a newly elected conservative President, Lee Myung-bak, took office.
He made it clear that the policy of engagement would no longer be unconditional, insisting that the North get rid of its nuclear bomb, and improve its human rights, as a precondition for further aid and trade.
Nine months on and inter-Korean cooperation is back in deep freeze.
The most prestigious cross-border tourism project, the Kumgang resort complex on the North Korean east coast, has already been shut down after North Korean border guards shot dead an unarmed South Korean tourist in July.
Now, as well as the closure of the daily bus tours, North Korea has cut off almost all dialogue with the South, severing the telephone hot-lines as well as the only rail link, a freight train supplying a joint industrial zone.
The factory complex, close to the city of Kaesong, will also have to make do with fewer South Korean managers as the number allowed to travel across the border each day is to be curtailed.
So who is the real culprit in the current crisis?
The easy explanation, and the one put forward by Pyongyang of course, is that South Korea is entirely to blame.
The new administration in Seoul has seriously underestimated the North's willingness to put pride before economic advantage, the theory goes.
Rather than comply with the calls for faster nuclear disarmament and reform, a deeply angry North Korea seems perfectly willing to sacrifice the tens of millions of dollars it earns each year from the joint tourism projects.
Cargo trains will no longer run from South to North Korea
President Lee's political opponents take this view.
The main opposition Democratic Party has condemned what it calls "the continuation of the hawkish stance" and says Mr Lee must change his policy "before it is too late".
They want him to honour without precondition the previous agreements for further economic cooperation signed by his liberal predecessors.
But as always in Korean affairs there are plenty of credible observers ready to point the finger in entirely the opposite direction.
They suggest that North Korea has never been comfortable with the mini-experiments in free market capitalism and bourgeois tourism trickling over its borders.
They have meant that 33,000 North Korean workers in the industrial zone have come to know their South Korean bosses.
And they've discovered that those bosses are not ogres or imperialist lap-dog stooges, but intelligent fellow Koreans with a standard of living they can't even dream of.
The North Korean citizens of Kaesong, a city without a private car in sight, witness the well-dressed visiting South Koreans in air conditioned coaches.
"At some point North Korea will face a tipping point and its elites will realise that it has failed," says Peter Beck, a professor at American University in Washington who specialises in Korean affairs.
"Engagement reinforces this fact."
So, the theory goes, North Korea is simply using the policies of new conservative government in Seoul as an excuse to disengage.
But perhaps there isn't really a crisis at all.
North Korea could simply be playing diplomatic hard-ball as usual, driving a wedge between allies by isolating South Korea while simultaneously trying to get closer to Washington as President-elect Barack Obama prepares for office.
The increasingly chilly relations, according to this explanation, are simply about calculated advantage, and could thaw again as quickly as they freeze.
As always with North Korea there is much speculation and very little hard fact.
Optimists point out that the new border restrictions stop short of a complete closure of the Kaesong industrial zone - at least for now.
The only thing that can be said for sure is that the uncertainty cannot be good for the nearly 90 South Korean companies that have set up business there.
And whatever its motivations, North Korea's recent actions have highlighted the dangers of too starry-eyed an approach to cross-border engagement.
In a recent article in the Korea Times, journalist and author Michael Breen raises the issue of political risk, given that investors, tourists and Southern politicians alike are subject to such unpredictability.
"You would think that only a raving lunatic would risk wealth and reputation dealing with North Korea," he writes.
"But it is surprising how many smart and well-meaning people get seduced into parting with them."