And the Syrian targets despite what you may hear cannot not be folded up and put into a hardened bunker or some cave somewhere. Syria can hide its men, but it cannot fold up airports and stick them in some cave. They cannot fold up their missile launch sites and stuff them into some cave either. Those military sites and bases are not going anywhere and will remain vulnerable to American attack. And there are only so many men you can stuff in a cave and keep indefinitely. Those underground men need food and water and other supplies.
A little told story in the Vietnam war is how most of the guerrillas hiding underground for long periods- frustrating as they were for American troops to deal with- ended up dying or debilitated from horrible diseases and malnutrition. Only the kind of life you'd want to live if you believed the alternative was certain death. As it's shown in Afghanistan and elsewhere, the US has plenty of tools in its bag to make sure that whatever goes into the caves never comes back out.