ISIS
The group originated as Jama'at al-Tawhid wal-Jihad in 1999, which pledged allegiance to al-Qaeda and participated in the Iraqi insurgency following the March 2003 invasion of Iraq by Western forces. Joining other Sunni insurgent groups to form the Mujahideen Shura Council, this group proclaimed the formation of the Islamic State of Iraq (ISI) in October 2006.
Not fighting back against radical islam allowed them to regroup. Donald Trump did not learn from 9-11, that is why he said and still says it was a mistake.