Regardless of the position you might have taken over the Iraqi war, it happened and in essence, the "coalition of the willing" took upon itself the responsibility of governing Iraq. It did so because Saddam Hussein controlled Iraq was seen as being a threat to its neighbours and more generally. Removing the regime was to enable Iraq to become a friendly and stable friend to Western interests.
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, 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.
Iraqi President at the time, Nouri al-Maliki, bears some responsibility for the disaster, but the overwhelming responsibility lies with the US State Department, British Foreign Office and those of the "coalition of the willing", for simply they were the coalition of the unwilling.
The overthrow of Saddam Hussein created a vacuum, that the "coalition of the willing" were unwilling to fill - that is of strict law and order, border control and to establish a government that would apply the rule of law, objectively, and defend the rights of all citizens. It's hardly surprising, for the sheer volume of forces needed to do that were beyond the willingness of any governments to provide (or afford). Unlike Japan, which culturally was in a sense of shock and fear after its defeat, and Germany which saw occupation by powers already spending vast proportions of their GDP (and were near neighbours), Iraq was flooded with weapons, full of thugs who lost their power after the fall of Hussein, and had porous borders with a neighbour that sought to make it compliant.
Iraqi Kurdistan is the shining exception.
The United States and its allies let Iraq down. After the success of the late surge, it let a bigoted corrupt sectarian leader take over the country, and as a result those who he was bigoted against, and excluded, turned on the regime, and found allies, and the genesis of ISIS was created.
It is very easy to be introspective, and say the original war was a mistake, and to blame Iran for its remarkable efforts at destabilising Iraq (notice how the anti-war movement in the West has absolutely no issue with Iranian imperialism, as the anti-war movement is, in fact, a movement against Western civilisation and capitalism). That effort is for academics, what policy makers need to consider is what to do now, particularly as ISIS is spreading, virus like across Syria and Iraq.
Iraqi Kurdistan is far from perfect, but there is a reasonable degree of rule of law, peace and governance that is far from disagreeable in the region. It effectively is part of a separate federal unit from the rest of Iraq, and deserves to be defended, particularly given the relative acquiescence of the world after Saddam Hussein's chemical weapon attacks upon them in the 1988.
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?