Who Are the Syrian Rebels Who Seized Aleppo?

Syrian rebels shocked the country with a lightning offensive on its biggest city and the surrounding region starting last week, upending a 13-year-old civil war that had been dormant for several years. By seizing that city, Aleppo, occupying large stretches of the countryside and advancing on the western city of Hama, the rebels have been able to expand their territory to include much of the northwest corner of Syria. And they are still on the march.

The fighters opposed to President Bashar al-Assad’s autocratic rule always included a motley patchwork of rebel factions who were often at odds with each other. But this time, the rebels have united under the leadership of Hayat Tahrir al-Sham, a former Al Qaeda affiliate that broke with the older group years ago and came to dominate the last stronghold of Syria’s opposition.

Once seen as one of the rebellion’s most powerful extremist factions, the group later tried to play down its radical aspects and focused on building something like a civilian government — albeit an authoritarian and extremist one — in its patch of territory.

Now the group may extend its control to a much larger zone. Here’s a primer on how it formed, how it evolved and why it staged this attack.

How did Hayat Tahrir al-Sham start?

Hayat Tahrir al-Sham, whose name means Organization for the Liberation of the Levant, began around the same time as Syria’s civil war, which broke out in 2011 after Mr. al-Assad’s troops violently suppressed widespread antigovernment protests. As the war intensified, experienced jihadists linked to a precursor of the Islamic State crossed from Iraq into eastern Syria, forming what they called Al Nusra Front to fight pro-Assad forces.