No one can say absolutely what the out come would be, but doing nothing is not going to lessen their threat on these shores.
Accurate attacks to disrupt their oil supplies/money, training camps etc will degrade their capabilities.
As for the aftermath, I doubt how effective attempts at a polical settlement will be. But if it's only groups of locals battling it out for a slice of the action in their own country, without exporting terror, then fine, leave them to it.