So yesterday was the burning of books. First came the looters, then the arsonists. It was the final chapter in the sacking of Baghdad. The National Library and Archives a priceless treasure of Ottoman historical documents, including the old royal archives of Iraq were turned to ashes in 3,000 degrees of heat. Then the library of Korans at the Ministry of Religious Endowment was set ablaze.

I saw the looters. One of them cursed me when I tried to reclaim a book of Islamic law from a boy of no more than 10. Amid the ashes of Iraqi history, I found a file blowing in the wind outside: pages of handwritten letters between the court of Sharif Hussein of Mecca, who started the Arab revolt against the Turks for Lawrence of Arabia, and the Ottoman rulers of Baghdad.

And the Americans did nothing. All over the filthy yard they blew, letters of recommendation to the courts of Arabia, demands for ammunition for troops, reports on the theft of camels and attacks on pilgrims, all in delicate hand-written Arabic script. I was holding in my hands the last Baghdad vestiges of Iraq's written history. But for Iraq, this is Year Zero; with the destruction of the antiquities in the Museum of Archaeology on Saturday and the burning of the National Archives and then the Koranic library, the cultural identity of Iraq is being erased. Why? Who set these fires? For what insane purpose is this heritage being destroyed?

When I caught sight of the Koranic library burning flames 100 feet high were bursting from the windows I raced to the offices of the occupying power, the US Marines' Civil Affairs Bureau. An officer shouted to a colleague that "this guy says some biblical [sic] library is on fire". I gave the map location, the precise name in Arabic and English. I said the smoke could be seen from three miles away and it would take only five minutes to drive there. Half an hour later, there wasn't an American at the scene and the flames were shooting 200 feet into the air.

There was a time when the Arabs said that their books were written in Cairo, printed in Beirut and read in Baghdad. Now they burn libraries in Baghdad. In the National Archives were not just the Ottoman records of the Caliphate, but even the dark years of the country's modern history, handwritten accounts of the 1980-88 Iran-Iraq war, with personal photographs and military diaries,and microfiche copies of Arabic newspapers going back to the early 1900s.

But the older files and archives were on the upper floors of the library where petrol must have been used to set fire so expertly to the building. The heat was such that the marble flooring had buckled upwards and the concrete stairs that I climbedhad been cracked.

The papers on the floor were almost too hot to touch, bore no print or writing, and crumbled into ash the moment I picked them up. Again, standing in this shroud of blue smoke and embers, I asked the same question: why?

So, as an all-too-painful reflection on what this means, let me quote from the shreds of paper that I found on the road outside, blowing in the wind, written by long-dead men who wrote to the Sublime Porte in Istanbul or to the Court of Sharif of Mecca with expressions of loyalty and who signed themselves "your slave". There was a request to protect a camel convoy of tea, rice and sugar, signed by Husni Attiya al-Hijazi (recommending Abdul Ghani-Naim and Ahmed Kindi as honest merchants), a request for perfume and advice from Jaber al-Ayashi of the royal court of Sharif Hussein to Baghdad to warn of robbers in the desert. "This is just to give you our advice for which you will be highly rewarded," Ayashi says. "If you don't take our advice, then we have warned you." A touch of Saddam there, I thought. The date was 1912.

Some of the documents list the cost of bullets, military horses and artillery for Ottoman armies in Baghdad and Arabia, others record the opening of the first telephone exchange in the Hejaz soon to be Saudi Arabia while one recounts, from the village of Azrak in modern-day Jordan, the theft of clothes from a camel train by Ali bin Kassem, who attacked his interrogators "with a knife and tried to stab them but was restrained and later bought off". There is a 19th-century letter of recommendation for a merchant, Yahyia Messoudi, "a man of the highest morals, of good conduct and who works with the [Ottoman] government." This, in other words, was the tapestry of Arab history all that is left of it, which fell into The Independent's hands as the mass of documents crackled in the immense heat of the ruins.

King Faisal of the Hejaz, the ruler of Mecca, whose staff are the authors of many of the letters I saved, was later deposed by the Saudis. His son Faisel became king of Iraq Winston Churchill gave him Baghdad after the French threw him out of Damascus and his brother Abdullah became the first king of Jordan, the father of King Hussein and the grandfather of the present-day Jordanian monarch, King Abdullah II.

For almost a thousand years, Baghdad was the cultural capital of the Arab world, the most literate population in the Middle East. Genghis Khan's grandson burnt the city in the 13th century and, so it was said, the Tigris river ran black with the ink of books. Yesterday, the black ashes of thousands of ancient documents filled the skies of Iraq. Why

Robert Fisk: Library books, letters and priceless documents are set ablaze in final chapter of the sacking of Baghdad