What led to Syria’s 13-year civil war, and why has fighting surged again?

Syrian opposition forces made a shock advance across the northern part of the country in recent days, seizing control this weekend of most of Aleppo, Syria’s economic capital, in a stunning challenge to President Bashar al-Assad that has refocused global attention on the nation’s years-long civil war.

Since its start in 2011, when Assad cracked down on largely peaceful protests, the conflict has killed hundreds of thousands of people and displaced around 13 million more, according to the United Nations. It sucked in world powers, including Russia and the United States, and carved Syria into different zones of control. The power vacuum left by the civil war also helped fuel the rise of the Islamic State.