Trump says Asad used chemical weapons against innocent civilians. Very unlikely that for some reason Asad picks this moment in time to use illegal weapons against his own civilians minding their own business. Asad may have used chemical weapons against USA backed rebels. that sounds a lot more reasonable. Trump took exception to that and that's his choice. As opposed to Gulf I when USA backed the kurds in Iraq, and then negotiated a cease fire, leaving the kurds to get slaughtered, at least Trump stands up for his mercenaries.
Asad has an 80% approval rate in Syria (Syrian told me this). He had to give a Mediterranean port to the Russians otherwise he'd be dead already. Syria is a stalemate. nothing to be done there. Asad paid what it took to get the Russians in to defend him, and they can bicker all they want, they're not going to get anywhere.