I believe the international community has made a mistake in not challenging the rise of the Islamic State "caliphate" (formally ISIS) at an early stage. The fact that the UK and USA made a mistake over Iraq is not really relevant; they might have sown the seeds of this chaos, but it doesn't mean that any subsequent intervention is wrong or likely to be similarly counter-productive - each case needs to be taken on its own merits. It must have been pretty clear early on that the Iraqi government was impotent (and not helping the situation by being exclusive) and did actually need some help through direct intervention. There must be ways of intervening directly, but minimising western casualties, whilst supporting Iraqi forces to defend themselves, otherwise our armed forces must be pretty unimaginative!
ISIS is clearly an evil force and cannot ultimately win. Distracting our law enforcement and border agencies by stopping fundamentalists from going to Syria/Iraq and fighting for ISIS is simply brewing up trouble at home. Most terrorism is home-grown - remember the Red Brigade, the Bader-Meinhof gang, IRA, PLO, etc - so these people are already radicalised. Let them go; they will most likely die and, if not, then should not be allowed to return.