However, although it essentially let the Kurds govern themselves, it failed - utterly miserably - to establish law and order in the rest of Iraq, with a government that represented and granted rights and rule of law across all Iraqis. Over 90% of those killed since the original invasion were at the hands of sectarian militia groups. The sectarian Shi'a administration now in Iraq, is relatively weak and is one reason why some Sunnis have embraced ISIS in resistance to that government.
However, the case for Western military intervention is much greater than being "nice" to the Kurds, there is a case of self interest here. A failure to take on ISIS and defeat it has the very real chance of being dangerous not only to those the West once called its allies, and many thousands (and millions) of innocent civilians, but a base for terrorist action in Europe.
If ISIS captures Baghdad, with no significant Western military effort, it presents the possibility that Iraq will see genocidal actions against Kurds and Iraqi Shi'a, that ISIS will turn on Kuwait and use the oil wealth of Iraq to fund further expansionism. It presents the possibility of Iran invading Iraq to prevent this, and without a doubt, such a takeover would be a clear indication that the West not only has abandoned the Middle East, but is willing to let ISIS have virtually free reign in its holy mission to establish a totalitarian Islamist Caliphate in the Middle East.
Some will say so what? They'll say so what until an ISIS bomb explodes in the Coliseum, or maybe St. Paul's Cathedral in London, or Notre Dame in Paris.
ISIS has clear intent to go beyond Iraq and Syria. It has embraced eliminationism with its conquests, demanding that people convert and submit, or get killed. It has murdered children, taken women and young girls to be sex slaves and beheaded and otherwise executed men for any form of dissent, including being gay. It seeks to eliminate Israel, to eliminate all of the hereditary monarchies in the Middle East, to make every Muslim dominated country into a dictatorial caliphate, and to expand this wherever it can.
So this is a terrorist group, seeking to establish governments, as a death cult, that celebrates when it commits genocide, that seeks to wipe out liberal secular democracy and wipe out civilisation in favour of its misogynistic Islamist pre-enlightenment nightmare. It has access to oil as a source of revenue and is unafraid of using the technology developed under civilisation to turn against it. It is the Taliban, with oil, with the expansionist interest of Nazi Germany.
The question is not if, but when there is inevitable conflict and if there is to be conflict, whether it will be with Arab and non-Arab allies, or whether the continued near isolationism of the West means that the Kurds, Iraqis, Syrians and others are just to be left to be slaughtered.
So what should be done?
ISIS should be attacked, first in Iraq, with the Iraqi military, as part of a concerted effort to recapture all territory from ISIS, attack bases inside Syria and secure Iraq from ISIS - which must also include the borders and ensure Iraq's government is of Sunni and Shi'a Muslims. However, once Iraq is secured, the decision must be made to go into Syria and eliminate ISIS. Yes, it will help the Assad regime, but it is not for that regime - and indeed a no-fly zone should be established to stop the Assad regime's use of chemical weapons and barrel bombs. It is to remove the virus of ISIS, it is to lance this boil before it establishes itself with power to project itself more than across immediate borders.
The US should lead this, with NATO and the Arab League, and Iran - yes, Iran. For Iran is positively moderate in comparison. For ISIS is a common foe, including a foe for Russia and I suspect eventually, China. Defence of Iraq does not need a UN Security Council resolution, just support from the Iraqi government, but beyond that attempts should be made to get multilateral endorsement. Yet that should not be considered a barrier to intervening by whatever means is necessary, to wipe out ISIS.
For if this is not done, there will be innocent victims, not just in Iraq or Syria, but in Europe and the United States and beyond. The Western leaders who are in charge if or when that happens, can hardly have been surprised, but should it take such a loss for action to be taken?