To show that Trump is a pawn of Putin, of course!
They lost nothing when the last, feckless leader of our country turned his back on Assad's last use of WMD. Russia will see to it that he doesn't run out of weapons and ammunition.
Putin said that this damages US-Russian relations. I think he's surprised that Trump The Bombastic has a backbone. Trump's increasing the percentage of the military in his inner circle of voices that he listens to. These are very dangerous times. Syria, Iraq, North Korea, and the happy horseshit that Russia's been pulling in Central Europe are all areas that the US has been pushed around for quite some time, and never as much as when the limp-wristed B.O. was in office. He, Hillary, and the stooge Kerry were absolute foreign policy disasters.