Aleppo, Aug. 13 -- Members of the al-Ali family were walking home from shopping when several shells slammed into the busy street on the western edges of the Syrian city of Aleppo. The blast tore through them, killing 2-year-old Salam and one of her cousins, and incinerated a car nearby with a woman and her infant daughter inside.

It was one of multiple attacks by rebels firing from Aleppo's outskirts that killed more than a dozen civilians last month.

Nearly three years have passed since President Bashar Assad's forces gained full control of Aleppo, sweeping out rebels who had held the eastern half of the city through years of fighting.

That victory made Aleppo - Syria's largest city - a symbol of how Assad succeeded with crucial assis...