As anyone who has seen Charlie Wilson's War will know, the teeming Pakistani city of Peshawar was the CIA's chief staging post in its supply of guns and money to the Mujahidin who drove the Red Army from Afghanistan. Its main bazaar became famous not only for rubies from the Hindu Kush, but also for Stinger surface-to-air missiles from the United States.

Two decades on, Peshawar still controls northern Pakistan's main route to Kabul via the Khyber Pass. It serves as gateway to the Federally Administered Tribal Areas along the western border. And it is under siege by the Taleban.

As we report today, militants who owe their allegiance to Islamist extremism rather than the elected authority in Islamabad have converged on this critically important city from both north and south. By controlling arterial routes in and out of the city they are able to cut off access and communications at will. Their harassment of business owners, so familiar from Afghanistan, has begun, and local police speak dismally, in the language of more traditional wars, of the imminent “fall” of Peshawar.

Strategically and symbolically, this would be disastrous. To lose control of Peshawar would be an incalculable blow to the fragile prestige of Pakistan's new Government, which has already gambled rashly in power-sharing deals with its restive provinces. It would pose an immediate new threat to Nato forces in Afghanistan, and a broader one to the enemies of terrorism globally: after seven years confined to Pakistan's wilder borderlands, al-Qaeda in Central Asia would effectively have the freedom of a main city.

Pakistan is already an anomaly. Without being labelled a failed state, it comprises large swaths of territory where its central Government's writ does not run and its forces dare not deploy. This has long been the case in the tribal areas. Since the installation of Prime Minister Yousaf Raza Gilani's new administration in April, similar arrangements have been reached in Swat, where troops have been withdrawn and Sharia has largely replaced Pakistani law; and in South Waziristan, where the power vacuum has been filled by Baitullah Mehsud, leader of the Taleban faction that is now encircling Peshawar.

Officially, Islamabad has not ceded control of Peshawar to anyone. Yet the Taleban are already closing down music shops and internet cafés. Those business owners who can are leaving. Last week 16 Christians were kidnapped from the city's main hospital. They were released hours later, but the militants had long since shown how threadbare Islamabad's authority has become: in February, they performed the same trick with Pakistan's Ambassador to Kabul.

Mr Gilani and President Musharraf have both used the same rationale for doing deals with separatist militants. It is better to appease them, they argue, than to wage an unwinnable civil war that could engulf the whole country. Yet where Pakistani forces have withdrawn, the Taleban has muscled in, with al-Qaeda in its shadow. Cross-border attacks into Afghanistan have more than doubled in frequency since last year, hampering both Nato's and President Karzai's nation-building efforts and making Islamabad look weak, not wise. Peshawar is where Mr Gilani must be strong. It is sovereign Pakistani territory, unambiguously and vitally. It must not fall.