By John Simpson
BBC News, Tora Bora, Afghanistan
Among the coalition's first targets in 2001 were the mountains of Tora Bora
The immense border between Afghanistan and the north-west frontier of Pakistan is harsh, inhospitable and breathtakingly beautiful.
It has been the cause of tension for at least a century and a half.
As "the Durand Line", the border was imposed on the Afghans by Britain in 1893. Even now, Afghanistan refuses to agree to it in principle, although, in practice, it is accepted.
Looking down from the Afghan side at Torkhum towards the Khyber Pass which leads into Pakistan, you can understand why stopping the movement of guerrillas and weapons across the border is so hard.
The road from the Khyber is the main trade route into Afghanistan, and is choked day and night with lorries packed high with Pakistani goods.
The border police on both sides try their best to check that guns and explosives are not hidden under the tons of onions or rice or electrical goods, but the job is an impossible one.
In Kabul, I interviewed a would-be suicide bomber from Pakistan who had given himself up when he realised his controllers had lied to him.
I asked if police had examined the lorry which he drove across the border laden with explosives. He shook his head.
The road from Kabul to Jalalabad and on to Torkhum is becoming more and more dangerous.
A year ago, when my team and I travelled along it, the police gave us an escort of a jeep containing four armed men.
This time we had eight jeeps and 48 armed men.
And when, a couple of days later, we drove southwards out of Jalalabad to the Tora Bora mountains, close to the Pakistani border, the Afghan authorities insisted on giving us a 14 vehicle escort.
On the dirt roads and mountain tracks which lead to Tora Bora, the biggest threat is landmines.
The Taleban who operate here cannot have failed to see our convoy, and would have guessed that we had to return this way. It would have been simple to lay mines in our path during the night.
Tora Bora means the black widow. It lies in the shadow of the Spin Garh (white-headed) range, which is covered with snow all year round.
The sight is breathtaking - fierce, brooding and impenetrable except on foot.
The Afghan border police have a hilltop position looking up at Tora Bora.
At the mountain's foot lies a narrow valley leading to the famous caves where Osama bin Laden hid, and eventually escaped from, in 2002.
Could we go there, I asked the police commander? No, he said, they were in no-man's land.
The Taleban, who have been forced out of the caves twice by coalition and Afghan troops, have now established themselves back there again.
From time to time the police fire heavy machine-guns and mortars across at Tora Bora, to assert their presence.
There was no return fire; the Taleban are too wary for that.
During the night, as we slept under the stars, the police were vigilant. Occasionally, they called out softly to one another to show they were still there, and that their throats had not been cut by marauders.
Yet the base is easily infiltrated. At one point after dark a fully-grown wolf loped across the open space in front of us.
In the morning the commander decided we should return by a different route - until he was told that an American convoy had been ambushed there nine days ago.
It was plain he was pretty anxious.
Then he and his men consulted their maps. By driving along a couple of dry river-beds we could curve round and join the main Jalalabad road after three hours' hard driving across country.
It worked. As we left the Black Widow's shadow there was no ambush.
Maybe the fact that we had an escort of 80 well-trained policemen, all armed with AK-47 assault rifles, had something to do with it.