To the outside world , they 're a force of ruthless yet mysterious insurgents bent on terrorizing civilians and expanding Islamist rule . But as one former ISIS fighter tells CNN , the mentality goes much deeper . `` The main and principal goal of the Islamic State that they tell their new members is to establish an Islamic state that will encompass the Arab world , '' the man said in Turkey . `` And after that , we go to other countries . '' Just two weeks ago , the man was in Raqqa , the ISIS stronghold in northern Syria . Like many cities across Syria and Iraq , Raqqa has been overrun by ISIS militants who show no mercy for those who do n't follow their hard line . Crucifixions and public executions take place virtually every day . Women not fully covered in niqabs are lashed with alarming frequency . Even store owners who leave their shops open during times of prayer can get beaten or sent to prison . The former insurgent , in his 20s , left ISIS two weeks ago because he said the group is `` spreading injustice in the name of justice . '' But he still agrees with the ideology of the group that is forcing its beliefs on everyday life , including in education . `` Philosophy is prohibited ; they canceled it as a kind of blasphemy , '' he said . `` Many subjects have been canceled , like music and even sometimes sports . All of them have been canceled from the school curriculum . '' Perhaps the only thing as disturbing as ISIS ' terror rampage is the growing number of foreigners joining its ranks . Thousands of foreign fighters are estimated to have joined ISIS . And the ex-ISIS militant said these foreigners could carry out attacks when they go home , but security measures in those countries could make it difficult -- at least for now . `` Since Western fighters joined ISIS , they consider their home country as infidels , '' he said . `` If they have a chance , they will carry out attacks . '' The United Kingdom is already on high alert , raising its terror threat level from `` substantial '' to `` severe '' on Friday . Prime Minister David Cameron has proposed a radical new measure to ban Britons from coming home once they join jihadi ranks abroad . And the United States got a harsh reminder of ISIS ' reach when Douglas McAuthur McCain , a 33-year-old from the Midwest , became the first known American to be killed while fighting for ISIS . U.S. Defense Secretary Chuck Hagel has called ISIS `` beyond just a terrorist group . '' `` They marry ideology , a sophistication of strategic and tactical military prowess , '' he said last week . `` This is beyond anything we have seen , and we must prepare for everything . '' ISIS may have wanted to show off its global reach by having a militant with a British accent front the videos of the beheadings of American journalists James Foley and Steven Sotloff . And that kind of tactic could also inspire more foreigners , the former ISIS fighter told CNN . `` It is possible that the goal was to project the image that a European , or a Western person , executed an American so that they can showcase their Western members and appeal to others outside Syria and make them feel that they belong to the same cause . '' Where do jihadis come from ? Opinion : Why we must all challenge ISIS . What is ISIS Achilles ' heel ?
ISIS plans to take over the Arab world and then `` go to other countries , '' an ex-fighter says . The man left the radical terror group two weeks ago . `` Since Western fighters joined ISIS , they consider their home country as infidels , '' he says .