Be careful who you chide for missing key parts when you're missing key parts, too.
ISIS formed in Iraq in 1999 as the group that would become al-Qaeda in Iraq. (They change names every so often, probably for media and PR reasons and because their goals change.) They were driven into Syria where they were able to regroup, rearm, and pull in the support of foreign fighters. Once they amassed enough power and made enough deals with Iraqi Sunni emirs, they crossed back and, using the support of various Sunni militias, drove the Iraqi military out of the region.
This isn't the complete story, either. It's very much worth understanding the socioeconomic and political pictures that have allowed the current situation to arise.