Timeline of Syria unrest

Key dates in the uprising against President Bashar Assad's regime in Syria, which entered its 18th month Wednesday: MARCH 2011 - 15: First protests after calls on Facebook for a "Day of Dignity." - 18-27: Crackdowns in Damascus, Banias and Daraa, cradle of the uprising, where 100 are reportedly killed on the 23rd. APRIL - 25-26: Protests spread and strengthen, with calls for the regime's fall. AUGUST - 18: US President Barack Obama and his allies urge Assad to quit. Western and Arab states later impose sanctions on his regime. OCTOBER - 2: Creation of the opposition Syrian National Council (SNC). NOVEMBER - 16: The Free Syrian Army (FSA), set up in Turkey by a deserter from Assad's army, claims an attack on a Damascus military base. MARCH, 2012 - 21: The UN Security Council demands that Syria implement a peace plan drawn up by UN-Arab League envoy Kofi Annan calling for a ceasefire and democratic transition. APRIL - 12: The UN-backed ceasefire, which the regime accepted, formally begins, but violence continues. - 14 and 21: Security Council Resolution 2042 -- the first adopted on the crisis -- calls for unarmed observers in Syria. MAY - 25: At least 108 people, including 49 children and 34 women, killed in Houla in Homs province. Rebels and regime blame each other for the massacre. The Security Council condemns Damascus. JUNE - 16: UN observer head General Robert Mood suspends the work of his mission because of escalating violence, although the monitors remain in Syria. JULY - 3: The US-based Human Rights Watch says Syria is holding tens of thousands of detainees in a "torture archipelago." - 6: Munaf Tlass, a high-profile general close to Assad, defects. - 23: Regime forces, boosted by elite troops, reclaim most of the capital after days of unprecedented clashes. - 18: Four top security officials, including Assad's brother-in-law Assef Shawkat, are killed in a Damascus attack. - 19: Russia and China veto for the third time a UN Security Council resolution that would threaten sanctions against Assad. - 28: The military launches a dawn assault on Syria's economic capital Aleppo, where clashes have been taking place since the 20th, using helicopter gunships, artillery and tanks as well as ground troops. AUGUST - 2: Annan resigns after five months of fruitless peace efforts. - 13: Rebels claim they downed a fighter jet. - 14: More than 23,000 people have been killed since the outbreak of the revolt, the Syrian Observatory for Human Rights says. - Former prime minister Riad Hijab, who defected on August 5, says that the regime is collapsing. Government forces concentrate their operations on the two main cities Damascus and Aleppo. - 15: A bomb explodes in central Damascus near the hotel used by the UN observer mission in Syria, wounding three people.