Profile: Aleppo, Syria's second city
Aleppo is Syria's largest city and the country's industrial and financial centre.
For the first year of the uprising against President Bashar al-Assad, Aleppo saw neither the large-scale protests nor the deadly violence that shook other towns and cities.
However, it suddenly became a key battleground in July 2012, when rebel fighters launched an offensive to oust government forces and gain control over northern Syria.
But the rebel advance was not decisive and Aleppo remains firmly divided into opposition and loyalist controlled sectors, with some parts of the city changing hands on a daily basis.
Opposition forces mainly control the east, while the government controls the west.
Since December 2013, pro-government forces have intensified the aerial bombardment of rebel-held parts of the city, causing thousands of casualties.
Aleppo, or "Halab" in Arabic, is one of world's oldest continually inhabited cities, being mentioned in Egyptian texts from the 20th Century BC.
Remains of a temple from the end of the third millennium BC have been found at the site of Aleppo's famous mediaeval citadel, which still dominates the area and provided a defensive stronghold for centuries.
Aleppo flourished politically and economically during the 18th Century BC as the capital of the kingdom of Yamkhad, until it fell to the Hittites.
Later, it became an important city of the Hellenistic period and a key trading post for merchants passing between the Mediterranean and lands to the east. It was eventually absorbed into the Roman Empire and then prospered as a hub for caravan traffic under Byzantine rule.
In 636 AD, Aleppo was conquered by the Arab Muslim troops. About 80 years later, during the rule of the Umayyad Caliph Sulaiman, its Great Mosque was built.
In the 10th Century, Aleppo became the capital of the northern Syrian Hamdanid dynasty, but it then suffered a period of war and disorder, as the Byzantine Empire, Crusaders, Fatamids and Seljuks fought to gain control of it and the surrounding region.
Aleppo did not recover until the middle of the 12th Century. Then, under Ayyubid rule in the 13th Century, the city enjoyed a period of great prosperity and expansion.
But this came to an abrupt end in 1260, when Aleppo was conquered by the Mongols. The city then suffered an outbreak of plague in 1348 and a devastating attack by Timur in 1400.
In 1516, Aleppo became part of the Ottoman Empire. It was soon made the capital of its own province and emerged as a nexus of trade between the Orient and Europe.
Aleppo's role as a transit centre for trade declined in the late 18th Century and was hindered further by France and Great Britain's demarcation of the borders of modern Syria - which cut the city off from southern Turkey and northern Iraq - and the loss of the Mediterranean port of Alexandretta to Turkey in 1939.
Following Syrian independence, the city developed into a major industrial centre, rivalling the capital Damascus, and its population expanded massively from 300,000 to about 2.3 million by 2005.
Today, Aleppo's population is made up mainly of Sunni Muslims, most of whom are Arabs but some of whom are Kurds and Turkomans. The city also has the largest population of Christians in Syria, including many Armenians, as well as Shia and Alawite communities.
War of attrition
When anti-government protests erupted across Syria in March 2011, the authorities did everything possible to ensure they did not take hold in Aleppo. The threat of brutal reprisals helped limit demonstrations largely to outlying districts of the city and the university.
However, as the uprising evolved into a conflict, Aleppo was soon caught up in the unrest.
In February 2012, the city was rocked by two bomb attacks on military intelligence and police compounds that left 28 people dead. Then, clashes between armed rebels and government forces began to be reported with increasing frequency in nearby areas of Aleppo province.
The battle for the city of Aleppo began in mid-July 2012. The rebels made swift gains, seizing control of several pro-opposition districts in the north-east, south and west.
By the end of the month, the fighting had intensified and spread to the historic city centre, even reaching the gates of the Old City, a Unesco World Heritage site.
In September 2012, a blaze swept through the ancient souk after clashes in the vicinity, while in April 2013 the Great Mosque's 11th Century minaret was reduced to rubble.
Within months the battle for Aleppo became a war of attrition. Today, most of the territory is disputed and the frontlines are shifting constantly.
The rebels have been unable to acquire the heavy weapons they need to defeat the better-equipped government's forces, while the military had its supply lines disrupted and had to focus resources on Damascus, where a rebel offensive also began in July 2012.
Since the end of 2013, government forces have waged a deadly aerial campaign in the city using barrel bombs, allowing them to make several gains.
Meanwhile, rebel forces in the northern city have been dogged by infighting between the Islamic State of Iraq and the Levant (ISIS) and other Islamist militant groups.
Experts say the battle for Aleppo is particularly important now ahead of the Syrian presidential elections on 3 June, and expect pro-Assad forces to try to wrest as much of the city as possible.