Now that the Islamic State of Iraq and Syria (ISIS) has conquered territory in both states, the US policy response is up for debate. We should, first of all, heed one of the major axioms of economics: in making a decision, ignore sunk costs, and consider only the future costs and benefits. The USA has spent huge amounts of treasure and sacrificed many lives, and also cost the lives and health of its allies and the people of Iraq. That is all in the past, and the US and other players should not make the mistake of being slaves to history.
One of the problems of US foreign policy has been that there is no unifying vision. The US seeks to defend itself from enemies, but it also claims to promote human rights and democracy, and it seeks to protect the status quo, current boundaries and governments. The US is also pursuing an aggressive foreign war on drug makers and users. Another policy goal is greater trade and economic development.
Another economic principle is that it is often less costly to prevent problems than to have to remedy them. The best foreign policy for the US is to, first, prevent the generation of enemies, and secondly, to defend against them when the enemies insist on that status. That proposition implies that US policy should avoid automatically protecting the status quo, and deal with the reality that exists.
The US has been fighting al-Qaeda because that organization has declared war against the US along with other countries, but we should not assume that all self-proclaimed Islamic regimes are necessarily at war with the USA. The problem in Iraq is that there are two clashing Islamic sects, Shiite and Sunni, and the US occupation set up a veneer of mass democracy that established a Shiite domination over the Sunni. That domination fuels an insurgency which now has been captured by ISIS.
It is probably now too late to restructure the governance of Iraq. Exhortations for greater inclusiveness are useless. Aiding the current government of Iraq would amount to taking sides in a civil war. The US should instead seek contact with the chiefs of ISIS and find out what they ultimately want. If they seek the destruction of the USA, then the US should defend itself now, before the ISIS becomes more powerful. But if they only seek to govern territory and re-establish a caliphate, and do not threaten other countries such as Jordan, then the US should monitor their activities but not become an ally of the Iraqi and Iranian Shiite governments in a religious war against ISIS. The US and its allies would then accept the fact that Iraq is no longer a unified country, but has split into three governments, the Kurdish region, the Sunni region, and the remaining Shiite-dominated land governed from Baghdad with the help of Iran.
The human-rights angle should still remain, as when the rulers become vicious, committing mass murder, then if that can be stopped, action would be warranted. But many regimes around the world are repressive and corrupt, and the US can do little about it other than to stop aiding them. The USA has its own violations of natural rights, and reform should start at home.