The wave of popular unrest sweeping the Arab world came late to Syria, but since the first protests in March 2011 in the city of Deraa at least 15,000 Syrians are thought to have been killed.
At the beginning of the uprising, demonstrators called for political freedom, an end to corruption and action on poverty.
However, after successive government crackdowns the conflict has become increasingly militarised, with government forces on one side and armed opposition groups - led notably by army deserters - on the other.
The opposition has come to control some isolated pockets of territory, particularly in the north-west of the country.
Many in the opposition now say their goal is the removal of President Bashar al-Assad and the ruling Baath party elite.
The government in Damascus says it is fighting "terrorists and armed gangs" and that 6,947 people have died, including 2,566 security forces personnel.
It also says it is facing an international conspiracy seeking to destabilise the country.
The Arab League suspended Syria from its ranks on 12 November 2011 and imposed sanctions. After weeks of tortuous negotiations on a peace plan with Damascus, Syria finally agreed to allow an observer mission from the League in.
But its monitors were criticised for failing to stop the violence, and Damascus dismissed the League's plan - modelled on the settlement reached in Yemen, whereby the president relinquished power to a deputy and left the country.
The observer mission has itself also come under attack when it has tried to access towns where massacres have been reported to have taken place. The observers suspended their work in January 2012.
In February and March, the rebel stronghold of Homs came under intense bombardment for weeks, with activists claiming hundreds of civilians were killed.
Kofi Annan was appointed envoy to Syria in March by the UN and the Arab League and put forward a six-point peace plan, including a ceasefire. A fresh observer mission under the aegis of the UN was also dispatched to the country.
However, that mission has also had to suspend its work in the face of continuing violence.
On 25 May reports emerged of the deadliest massacre in the crisis to date. UN observers confirmed that 108 people, most of them women and children, were shot or stabbed in the village of Taldou in Houla region.
Only a few weeks later, another massacre in the village of Qubair left 78 people dead.
With a leadership determined to cling to power, and a revolt that shows no sign of easing, correspondents say any resolution looks a distant prospect.
Turkey and Jordan have both called on Mr Assad to resign. The US and the EU have imposed sanctions on him and members of his government.
A meeting of world powers in Geneva in June called for a "transitional government", but Russia and China have blocked attempts by Western countries at the UN to put pressure on Mr Assad to leave.
Mr Assad had promised reform since 2000, when he inherited power from his father Hafez, but little had changed before the uprising began.
In recent months Syria has held a referendum on a new constitution, and parliamentary elections, hailed by the government as signs of progress. Opposition activists have dismissed the moves as cosmetic.
Events in Syria, one of Israel's most bitter enemies and a strong ally of Lebanon's Hezbollah militants, could have a major impact on the wider Middle East.