Assad runs a mass-scale torture state. Iran supported Assad from the beginning, without it he wouldn't have survived to 2015. The result of Assad's butchery is a mass displacement of Syrians, ergo refugee crisis, ergo Leave victory in Brexit referendum.
If the US was ever serious about not letting Assad and Iran get away with it all that wouldn't have happened, and there wouldn't have been Brexit. The US's decision to not get involved against Assad (they're there for ISIS I remind you) had a far worse result in human lives and geopolitical impact than any of the US's 'meddling'.