The scene at Pakistan's border with Afghanistan is depressing at the best of times. Most days a crowd of desperate Afghans try to cross the fairy castle-like gate, only to find themselves beaten back by Pakistani soldiers armed with sticks.
But since last Tuesday's attacks, thousands more Afghans have packed up their few meagre belongings and started trudging towards the unlovely border town of Torkham.
Some 5,000 people have arrived in the past 48 hours. More are coming every hour. A humanitarian tragedy of devastating proportions is unfolding.
Although there is no television inside Afghanistan, most Afghans know from listening to the radio that the threat of US retaliation is real - and imminent.
"In a situation like this you feel that death is creeping up on you as we don't know when the attacks will take place," one baker from Kabul said. "I am leaving with my family. We could not wait."
"We don't know whether we should run or hide," Morad Ali, a civil servant from Kabul, added.
Pakistan's Khyber Rifle border guards have been instructed to allow into Pakistan only Afghan refugees who have visas and proper documents. Most do not.
But it is impossible to hold back this human tide: Pakistan's 1,560-mile border with Afghanistan is extremely porous and easily breached.
Well-trodden routes exist over the mountains either side of Torkham, and elsewhere across the rugged, British-drawn Durand line. And in the past, for a 150-rupee bribe (£1.50) the guards would happily wave fleeing Afghans across. The price has now gone up considerably.
Search for food
There is, of course, nowhere else to flee to: Iran sealed its border with Afghanistan and stepped up patrols yesterday. Some 10,000 Russian guards are stationed in Tajikistan on Afghanistan's northern border. And Uzbekistan and Turkmenistan, which are both grappling with their own nascent Islamic revolutions, have also shut their doors.
With the US poised to launch a massive assault inside Afghanistan, senior aid workers warned last night that the country is heading for what could become the world's worst humanitarian crisis.
The World Food Programme said yesterday that its food stocks inside Afghanistan will run out in two weeks. The agency has been keeping more than three million poor Afghans alive: its women-run bakeries in Kabul churn out endless loaves of flat, chewy Afghan bread, and it distributes sacks of American wheat flour to remote villages.
When the food is gone, huge numbers of people will be forced to flee their homes in the search for food,the agency predicts.
And in when winter sets in, many villages will be cut off until spring.
"War would make it impossible for us to operate," one senior aid worker said last night. "It would multiply the worst humanitarian disaster by two or three times and render us totally ineffective."
Even before the threat of US retribution, Afghanistan was in the grip of a little-noticed catastrophe.
More than six million people were already in danger of starvation, most of them women and children. Afghanistan has not just suffered from two terrible decades of war and Soviet occupation, but has also experienced the worst drought in its history.
The drought has gone on for three years, withering crops, turning fields into dust, and killing livestock. Nearly one million Afghans are internally displaced. They have left their homes to escape the fighting - which still rages between the Taliban and the opposition forces - or have fled in search of food.
"I have just returned from Afghanistan and I cannot avoid a growing feeling of dread at what may be about to befall the people I have left there," Chris Buckley, Christian Aid's programme officer for Afghanistan, said last night.
The last foreign aid workers in Afghanistan pulled out yesterday under orders from the Taliban regime, as it digs in for war.
When the Taliban swept across Afghanistan in the mid-1990s, much of the country was in disarray. The Taliban made things worse by, among other things, persecuting the minority Shia Muslim population and laying waste to opposition areas.
In central Afghanistan, around the wrecked, beautiful town of Bamiyan, Taliban soldiers forced villagers from their homes. They destroyed summer pastures and chopped down orchards, leaving families with no shelter or food.
Some have found casual labour and have gathered enough firewood to keep them alive for now. But these stocks will not protect them from the winter.
There is, of course, nothing new about Afghan refugees: some 6.2m people have left since the Soviet invasion in 1979. Afghanistan boasts the world's largest refugee population: its diaspora stretches from Pakistan (2m refugees) and Iran (1.8m), to India, Australia, and Britain.
Those who make it into Pakistan, meanwhile, are not exactly welcome. Pakistan's military government has been attempting to get rid of a problem it believes has grown out of control. In November, it briefly sealed its border with Afghanistan.
The authorities in Peshawar, where most Afghans end up, have begun bulldozing the long-standing refugee camp of Nasir Bagh. Some 150 Afghans from the camp were herded onto buses and then dumped back across the border in Afghanistan, a wholly illegal act.
Some 80,000 Afghans are holed up in Jalozai, a refugee camp that has become a byword for desperation and squalor.
With a shadow of imminent disaster hanging over their homeland, they now appear to be the lucky ones.
A shopkeeper who had chosen to remain in Kabul said yesterday: "People are so tense. Any moment of calm is seen as the calm before the big storm.'
Other Afghans vowed to stay and fight. "We will go back and fight as we did with the Russians," Mohammed Ibrahim said. "We may have been fighting among ourselves, but when Afghanistan is under attack we will come together. We will unite."