The situation in Syria is rapidly changing, and it looks like the city of Homs is on the verge of falling into rebel hands. Rebel forces, led by the group Hayat Tahrir al-Sham (HTS), have been advancing south from their recent capture of the town of Hama. If they take Homs, it would be a huge blow to President Bashar al-Assad's regime, which has already lost the city of Aleppo in the last 10 days.

Homs is a key city in Syria. It lies just 75 miles north of the capital, Damascus, and is a...