At the time of the invasion, everyone new that the WMD justification was just a pretext. Iraq was complying within a very invasive inspection regime that was an unprecedented foreign intrusion on a sovereign nation. The intelligence reports about mobile biological weapons labs was such an obvious fabrication that it was shocking that our intelligence community wasn't resigning en masse in protest.
The real reason for the invasion of Iraq was to tip the balance of power among Sunni's to Saudi Arabia. Saddam's Baathist movement was actually very progressive compared to the Saudi's Wahhabi version of Sunni Islam. Women in Iraq could hold jobs alongside men and could participate in government. The fear was that Saddam would be successful in reforming Iraq by complying with UN weapons inspections and would become an alternative to the Saudi center of power in the Middle East. Saddam was not a US client, but Iraq was starting to improve relations with France before the second gulf war.
The idea of regime change was attractive considering the recent history of US intervention. Grenada, Panama and Serbia/Kosovo were all cases where the US exercised military power with minimal casualties and gained a stable alternative client state with little post-conflict effort. The Bush administration believed that the group of ex-pat Iraqi clowns lead by Ahmed Chalabi would be welcomed by the Iraqis as liberators.
In order to lay the ground work for that transition, L. Paul Bremer order the dissolution of the Baath party and the Iraqi army so the US could install a client government lead by Chalabi and his cronies. The response by the Iraqi people was immediate and catastrophic. Iraqi Army weapons depots were raided. Baath party and Iraqi army leaders went underground to plot their return to power. The Shia majority began its revenge campaign against the Sunni minority that ruled it with an iron fist under Saddam. The US installed government was corrupt from the start and never had the trust of the Iraqi people.
ISIS was the vision of Wahhabi Muslims who wanted to form a very extreme Islamic caliphate that was completely free of any relationship with the West. It is thought to have been historically tied to the Muslim Brotherhood as an extreme Islamic conservative opposition movement to governments in Saudi Arabia and Egypt, which were clients of the US. ISIS rose to power by drawing upon the talents of former Baathist and Iraqi Army generals. Unlike Bin Laden's followers, who were mostly the product of Madrasas with limited battlefield experience as guerrilla fighters in the Afghan/Soviet war, ISIS had very sophisticated military leaders and former government officials who were able to out fox the feeble Iraqi defense forces and easily take over Sunni territory that was resentful of the Shia dominated Iraqi government.
In all fairness, no one expected the former Baathists to align themselves with followers of a very extreme Wahhabi sect. But it was very much foreseeable that the former Baathist would go underground and form a counterinsurgency. many called for a truth and reconciliation process to keep the existing Iraqi army and government in place while transitioning to a new democracy. That would have enable the Iraqis to secure all their weapons and keep an insurgency from forming.