When the Syrian government dropped what the U.S. says were chemical munitions this month in the northwestern province of Idlib, it was targeting a rebel bastion that it helped create.
For months, the Russian-backed Syrian army had been recapturing Aleppo and other major cities — and giving the defeated rebel fighters safe passage to the province.
Through reconciliation deals, roughly 31,000 rebels and 75,000 of their relatives and other civilians have wound up there. Idlib became the main bastion of resistance to President Bashar Assad.
The province has also been a top destination among the millions of people who have been displaced in the civil war,...