The city of Herat in western Afghanistan would be a huge gain for the opposition National Alliance if it was captured from the ruling Taleban.
The ancient oasis, 150 kilometres (100 miles) from the Iranian border, is one of Afghanistan's five largest cities.
It is also a vital transit and goods route and home to one of the largest military airfields in Afghanistan.
Retaking it would open the way for the opposition to approach the southern city of Kandahar, the power base of Mullah Mohammed Omar, the Taleban's supreme leader.
Breadbasket
Herat, the city considered to have the most fertile soil in central Asia, was first settled 5,000 years ago.
The ancient Greek historian Heroditus called it the breadbasket of the region.
"The world is like an ocean," it was said in ancient times, "and in the ocean is a pearl, and the pearl is Herat."
When Alexander the Great came to Herat in the 3rd century BC, the city was already a prosperous place.
Later it became the greatest of the cities of the ancient Persian kingdom of Khorasan.
Florence of Asia
The golden age of Herat was the time of the Timurids, in the 14th and 15th centuries, when the city was known as the "Florence of Asia".
The best painters, the best architects, the best musicians, all came from Herat.
"In Herat if you stretch out your feet you are sure to kick a poet," said Ali Sher Nawai, a statesman who was himself a poet and artist.
It was at this time that the beautiful palaces and mosques which still adorn the city were built.
Notable among these are the Musalla complex, built in the late 1400s by Queen Gawarshad.
The city is the burial place of Afghanistan's greatest mystic poet, Khaja Abdullah Ansari.
Herat was formerly also renowned for its bazaars - the city is a major carpet-making centre.
Violent past
It is also famous for its military history.
Herat's ancient fortress was fought over by Persians, Turks, Mongols and Uzbeks.
The city came under very heavy bombing by the Soviets shortly after their invasion in 1979, when Herat's population rose up in an unprecedented revolt, killing Soviet officers, advisers and their families.
With continuing US air raids and renewed fighting in the region, there are growing fears about the preservation of one of the most exotic cities in the world.
Anti-Taleban feeling
The opposition troops fighting at Herat are said to be led by Ismail Khan, a Northern Alliance warlord and the city's former governor.
Mr Khan said last month that there was little admiration for the Taleban in Herat.
Mr Khan's forces had been driven out of Herat by advancing Taleban fighters, and he spent three years in prison before escaping to join the Northern Alliance.
"For nearly a year now, people have hated the Taleban because of their cruelty," he said. "Also, they've failed to come up with any coherent policy. They have failed to rebuild or improve anything in Afghanistan, and they have abused Islam to abuse their own people.
"People have grown very wary of them - and I can say that about 80% of people have turned their backs on the Taleban."