As ever, what the US really wants is to have a dominant position in post-war negotiations, so they can dictate the form that post-war Syria takes. This means ensuring that the Syrian government doesn’t win the war outright.
That’s not as realistic as it once was, with the Assad government, backed by Russia, having retaken virtually all of the rebel-held territory except for a far north bastion in Idlib, dominated by al-Qaeda. This means the US now has to save al-Qaeda to keep the war going, which if we’re being honest has been a recurring undercurrent in US policy in Syria for years.
It is this desire that has the US repeatedly threatening Syria and warning them not to attack Idlib. It is this desire that is sparking almost daily US threats to intervene militarily if the Idlib offensive involves chemical weapons. Most importantly, it is this desire that has Russia very much believing media reports that the rebels could “stage” a fake chemical attack just to suck the US into the war, and be fairly confident it would work.
The US is, after all, constantly talking about an imminent chemical attack despite there being no reason to think Syria is poised to launch one. At times, US officials have privately conceded that there is no sign Syria is making any moves to even ready such weapons for the offensive. Yet several times a week, the US issues statements with allegations of a chemical plot featuring prominently, setting the stage for a reaction.
The Syrian War has been nearing its endgame for months now, with Israeli officials conceding it is all but over as far as they are concerned (while vowing not to honor any post-war deals). When a war is lost and a plan has failed, however, the US government is often the last to know, and that has them determined to drag the war on as long as possible.