Like the Taliban or Al Qaeda, they wear normal clothing and can easily blend in with the rest of the civilians there. They can go into hiding and never be noticed in a crowd unless you know who you are looking for. They hide and fight in crowded urban areas making it is easier for them to escape. ISIS can travel very well because they can travel with migrants and never be detected while traveling from one country to the next. They are arriving to the Middle East from places like the Sub-Saharan Africa, Tunisia, and Europe (“What ISIS Really”).
Another reason they are dangerous is because the citizens believe that no other country will occupy their own country longer than ISIS, leading to a reason behind why many Muslims are joining ISIS. They know that this group is going to be there longer than any other country, so there interpretation of this is, if you can’t beat them, join them. They are afraid of what might happen if they oppose this group. They could be tortured or sent out with a bomb strapped to them to kill innocent people and themselves.