CBS News reports as many as 20,000 foreign fighters have traveled to Syria to join those fighting for the Islamic State, aka the Caliphate. At least 3400 are from the West. The article treats this as a problem.
It can, however, be viewed as an opportunity. By concentrating jihadis in one area far from us, it should make them easier to target and kill.
Both the Europeans and the U.S. should revoke the passports of those who leave our countries to join the jihad. Instead of a revolving door it should be a one-way trapdoor, a permanent, irrevocable choice to live in the Near East's medieval squalor and depravity.
Thus accomplishing what my elderly relatives called "good riddance to bad rubbish."