The Syria conflict is not new, what is it about this latest gas attack that's spurred him on? I'm not belittling this latest outage but scored of kids have been killed there before and he's publicly stated that the US shouldn't get involved.
I genuinely do not think the sight of dead kids has caused his retaliation, I think he's bombed the airbase to make it looks like he's not in Russia's pocket. I also think Russia are in on it with their faux outrage about an airstrip that doesn't harm them. It takes the heat off the Putin/Trump scandal.