The terror organization's rise in a tumultuous Middle East has provided Obama some of the toughest decisions of his presidency, choices that CNN's Fareed Zakaria explores in "The Legacy of Barack Obama" airing Wednesday.
"The ability of ISIL to not just mass inside of Syria, but then to initiate major land offensives that took Mosul, for example, that was not on my intelligence radar screen," Obama told Zakaria, using the administration's term for the Islamic State terror group.
