How to Understand Syria’s Rapidly Changing Civil War

Rebel groups fighting to depose President Bashar al-Assad of Syria battled regime forces on the outskirts of the strategic city of Homs on Saturday as they pushed toward the capital, Damascus, according to the rebels and a war monitoring group.

Advances by a coalition of opposition groups headed by the Islamist faction Hayat Tahrir al-Sham have abruptly changed the landscape of Syria’s civil war after a long stalemate. Their lightning offensive poses the most direct challenge to Mr. Assad’s power in years and is raising fears of chaos if his authoritarian government loses control over large swaths of the country.

The Syrian civil war started 13 years ago, beginning during the Arab Spring and escalating into a bloody, multifaceted conflict involving domestic opposition groups, extremist factions and international powers including the United States, Iran and Russia. More than 500,000 Syrians have died, and millions more have fled their homes.

Here’s a guide to understanding the conflict, even as it changes rapidly.

In just over a week, Syrian rebel forces have seized much of Syria’s northwest from the government in a fast-moving attack, upending the stalemate in the civil war. After capturing most of the major city of Aleppo last week, the rebels drove government troops from the western city of Hama on Thursday. They are now threatening the strategic city of Homs, and edging closer to the capital, Damascus.