What's happening in Syria? What to know about the complex, long-simmering civil war

A surprise offensive by rebel forces opposing Syrian President Bashar al-Assad's regime has led to a new round of fighting in the Middle Eastern country's long-running – but until recently largely static – civil war.

After a lightning advance in just a few days, rebels captured almost all of Aleppo, which is Syria's second-largest city. However, Assad in a speech Saturday vowed to defend the country's "stability and territorial integrity."