Not only was it sudden, but it was shockingly brutal! Since they represent the true Islamic community, anyone who resists them should be killed. So they have slaughtered thousands - mass executions of prisoners, crucifixions, beheadings, and rape are part of their repertoire.
Not only do they do these things, but they broadcast them through social media. In particular, they recorded the beheadings of two American journalists along with a message of defiance against American airstrikes in Iraq.
So what is this all about? Why are they so brutal and so exibitionist in their atrocities?
For one thing, they are modeling themselves on the Islamic movement under Mohammed and the original Calphs that followed him. What they are doing is pretty much identical to the approach of early Islam as they swept through the Middle East, across North Africa and Spain. The severe brutality of their treatment of conquered peoples tended to increase "conversions" and reduce opposition.
Another thing they are doing is recruiting people to their cause. For Moslems who live in secular countries (even the Moslem majority secular countries) this Caliphate is a promise of a return to the golden days of Islamic expansion and the promise of Islamic world domination. For them, this is the promise of a utopia where people all live under strict sharia law and justice for lawbreakers is swift and severe. For them the end justifies the means and participation in this fight is submission to God that will be rewarded in the next life.
This Caliphate is attractive to some who are jaded by American hyper-individualism and the constant pursuit of pleasure. The Caliphate offers community and a simple path to transcendent purpose in life - even if it requires brutality against the infidels.
How sad that American Christianity has been so absorbed into American individualistic "good times" culture that these nascent Islamists have not learned the true Christian path to transcendent purpose in life by following the call to love God; to love our enemies; to love our neighbors; and to love each other - as God has loved us in providing salvation through Jesus Christ.