Syrian government forces are advancing on Aleppo, aiming to besiege the rebel-held city into submission in what would be the biggest blow yet to the three-year uprising.

The fall of Aleppo, Syria’s largest city and economic hub before the fighting, could also bolster the ranks of Islamic State militants who continue to make gains across the country, as defeated rebels from the Western-backed Free Syrian Army switch to their side.

Rebel commanders in Aleppo say they’ve started to stockpile goods as aid groups step up food deliveries—crates of lentils, rice, ketchup and baby formula—seeking to prevent the same kind of mass starvation that forced them to surrender the much smaller city of Homs in May. […]