ISIS created themselves in Syria after fleeing Iraq. Then they bided their time until the moment was right to return to Iraq and begin retaking territory.
No doubt who gave them that moment: Barack Obama and his stupid policy of withdrawing all troops by the end of 2011 while failing to negotiate, or require that a sensible sized residual US force remain in Iraq.
And further, Obama failed to attack the ISIS Toyota convoys while they were moving in the open desert from Syria into Iraq, before they’d taken any territory. He screwed around for months before taking any action at all and then only limited air strikes.
After the Islamists who have committed the atrocities against minorities in Iraq, Obama is the person most responsible for all the murdered and displaced Iraqis. What happened was predicted.