Yes, there are many Iraqi Sunnis who are throwing in their lot with ISIS, but what's happening is mainly an invasion of Iraq from Syrian territory. Don't believe the spin that this is primarily a "civil war".
ISIS, which is no less dangerous to the United States than Al Qaeda (from which it grew), was allowed to fester and grow in Syria. Instead of choosing the lesser of evils in Syria (be it Assad or one of the rebel groups), and then backing them to the hilt, the U.S. instead decided to sit back and be content with removing chemical weapons from Syria. So ISIS grew and grew, and then they INVADED Iraq.
Iraq was free of these ISIS/Al Qaeda types when Obama came into office. And now they are on the verge of conquering Iraq, without the United States lifting a finger. This is extremely dangerous. Extremely.