An article from Open Democracy that starts to explain why Assad used / uses chemical weapons and offers an explanation of a civil war where nothing is simple. For once the few comments and the author's response are worth reading too.

I'd not spotted this:
In reality, there have been hundreds of chemical attacks in Syria reported since 2012 - including up to 85 merely in the past year since Trump’s ‘airfield strike’ of April 2017 (according to Human Rights Watch) - most of which only garner marginal media coverage and provoke little media and online commotion.
Or how territory changes "hands":
In reality, the military support provided by the US administration to the Assad regime, whether by allowing the influx of tens of thousands of fighters from the Popular Mobilisation Units (PMUs) backed by the Coalition in Iraq, or by returning former ISIS territories exclusively either to factions collaborating with the regime (the SDF) or the regime itself, has been infinitely more valuable.
(Later) Indeed, one of the biggest ironies of the conflict is that in the fight against ISIS, there have been more recorded occasions of the US-led Coalition supporting pro-regime militias than there have been of active anti-Assad groups.
Link:https://www.opendemocracy.net/north-africa-west-asia/omar-sabbour/why-would-assad-do-it-debunking-abstract-theories-surrounding-sy?