'MY enemy's enemy is my friend." It is an old saying, but it has never been truer than in today's war in Afghanistan.

Once fighters who are now with the Taliban were America's friends because, in the 1980s, they were fighting against the Russians who were then America's enemy.

In the early 1990s, the Northern Alliance became America's friends and were allowed to take power when the Russians left.

But then the Alliance became too brutal, and the CIA decided to befriend the Taliban. When the Taliban became too brutal, America decided it was the enemy and so turned to its old friends, the Northern Alliance, who had for a while been the enemy.

Just to make matters even more confusing, the Americans are being supported by their new friends, the Russians, who were once their enemies. And some members of the Taliban are defecting to join their old enemies the Alliance; others are being shot by the Alliance. Barbarity is replacing barbarity.

But, despite all this treachery, the Americans appear no nearer fulfilling their stated war aim of capturing Osama bin Laden. He's fled to the mountains, hoping to follow the example set by the Northern Alliance when they were turfed out of office, by pursuing a guerilla war.

Yet treachery will probably play its part here, too. Someone will blurt out his whereabouts, for 30 pieces of silver, and the Americans will have their man. What will they do next? Will they follow their time-honoured practice and bow out, only to become involved again with new friends when a new emergency situation arises?

To an American, it must be difficult to see what Afghanistan has to offer once all of its dust and hills have been bombed. Germany and Japan were heavily invested in after the Second World War, but that partly was because they had plenty to offer: they were a huge potential market for American products, they had huge skills and technology potential.

But the poor old Afghans have nothing tangible to offer, apart from dust and heroin.

The big test for George Bush is whether he sees beyond this, for the hills and caves of Afghanistan do, in fact, offer a vast prize: peace.

There will not be peace in the Middle East or the wider Muslim world until the Americans impose some sort of stability and order upon Afghanistan. And, until there is peace in the Middle East, America will always be at risk from spectacular acts of terrorism, as happened on September 11.

But once bin Laden is in a box, the temptation for President Bush, and for Tony Blair, will be to forget about Afghanistan. Already, there are pressures upon them to refocus on the domestic agenda. Already, some sections of the British media are questioning why Mr Blair has to fly around the world when the health service and education are still in crisis. But they must keep an eye firmly trained on Afghanistan.

Once the dust has settled, their influence will be huge. The Northern Alliance, for all its barbarity, knows it has only rolled back the Taliban because of America's efforts.

President Bush must not allow himself to forget that faraway on Shomali Plain outside Kabul he has the chance to make a real difference.