City under siege: the battle for Aleppo

Following the kind of bombardment which reduced buildings to rubble in other cities, notably Homs and Hama, infantrymen will then move into Aleppo's narrow streets to engage the FSA defenders, most of whom are armed only with personal weapons such as AK-47 assault rifles or shoulder-mounted rocket-propelled grenade launchers.

Some possess machine guns and mortars supplied by Saudi Arabia and Qatar, but this will always be an unequal battle and the casualties will be appallingly high. If the fighting is as bitter as it usually is in most civil wars there will be a bloodbath. Aleppo could be this century's version of the Battle of Stalingrad – the vicious struggle between the Red Army and Nazi forces in the winter of 1942-43 which claimed an estimated two million lives.

According to Navi Pillay, UN High Commissioner for Human Rights and a close observer of the fighting in Syria, a pattern has begun to emerge as Assad's forces resort to using terror tactics – shelling, tank fire and summary executions.

"All this, taken along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city," she said in a statement on Friday.

For the military commander defending or attacking Aleppo, the northern Syrian city provides a number of challenges. Its size is daunting, a vast sprawling area which covers about 74 square miles, the bulk of it is crowded and densely housed, meaning that the rival forces have to fight in built-up areas, a task which soldiers traditionally dislike.

As Assad's armoured columns and artillery approached Aleppo from the south, along the highway from Damascus, they were able to move quickly through the open suburban areas. Their problems will begin if the FSA fighters move into the central Citadel area, which is home to the 12th-century Great Mosque and madrassas, private residences and public baths.

With the narrow streets of its souk and the high walls of its buildings, the Citadel will be a nightmare to attack and it offers huge advantages to the defenders. From afar and from the summit which overlooks the sprawling city it is not dissimilar to Edinburgh and its castle – ironically it is, like the Scottish capital's Old and New Towns, a World Heritage Site.

If it becomes a battleground, it is not only lives that will be lost. An important part of the history and heritage of the Arab world will be gone. Aleppo lies at the western end of the old Silk Road and in its time – it was founded in the 2nd millennium BC – it has been ruled by the Hittites, Assyrians, Akkadians, Greeks, Romans, Umayyads, Ayyubids, Mameluks and Ottomans. Now it could be destroyed on the orders of Assad, an Alawite, traditionally one of the smaller and less influential of Syria's religious groupings.

However, Aleppo's history has nothing to do with the reasons for the decision to attack it. The country's major manufacturing city, it is home to industries including textiles, chemicals, pharmaceuticals and light engineering as well as traditional crafts such as carpets and the famous locally-made laurel soap. Aleppo represents the modern country and its loss would be a body blow for Assad.

The majority of the Muslim population are Sunnis, though Aleppo also contains a substantial Christian population, both Orthodox and Catholic. Although the local people were anxious to remain aloof from the civil war there are now signs that many of them, especially those in the populous eastern suburbs, have welcomed the FSA, providing meals for their fighters after sundown during the current holy period of Ramadan, when the faithful fast. Those inhabitants who simply do not want to get caught up in the fighting have left the city. All last week thousands of people, mainly women and children, made their way north towards the Turkish border to escape the violence that has been unleashed in Aleppo.

At the same time, the regime received a further blow when one of the city's provincial members of parliament and one-time supporter fled into exile in Turkey. Ikhlas Badawi, a mother of six, said she was defecting in protest at the "violence against the people". She said she could no longer stand by and watch the Assad regime slaughtering innocent people.

There is another reason why Aleppo is a vital target for both sides: It commands northern Syria and covers the routes into Turkey. With its size and its position, not to mention its religious make-up, it could fulfil the same role for the FSA as did the eastern Libyan city of Benghazi for the opposition National Transitional Council during the civil war to unseat Colonel Muammar al-Gaddafi last year.

Having been seized by the rebels and subsequently protected by their militias, Benghazi became a rallying point and the centre of a "safe haven". If the FSA manage to hold Aleppo, analysts – such as Riad Kahwaji, director of the Institute for Near East and Gulf Military Analysis – argue that there is no reason why it could not fulfil the same function.

"If you have Aleppo then virtually you have Idlib," Kahwaji said on Friday, referring to the region to the south-west of Aleppo now mostly under FSA control.

"So you're going to have all that enclave becoming a safe zone for the rebels, from where they will be able to organise, train, get all the arms needed for a major onslaught on the regime forces."

Equally, Assad has good reason for wanting to hold the city. If Aleppo remains in his hands, he has access to its industrial and commercial wealth at a time when Syria has been more or less bankrupted by 17 months of non-stop fighting – the regime is currently dependent on financial aid from Iran. Assad would also be able to maintain that he continues to enjoy the support of Syria's mainly Sunni population, an important factor considering that most of the FSA are of the same sect.

Above all, if his forces get the upper hand they can claim to be winning the war at a time when it appears that the rebels are beginning to scent victory. Before leaving Damascus at the end of last week, Major-General Robert Mood, the Norwegian commander of the UN monitoring mission, said that it was "only a matter of time" before Assad was removed.

That makes Aleppo doubly important: if government forces take the city, Assad can breath again. Lose it and his hold on power will be massively dented.