THE world is used to covert acts of violence by those who claim to be innocent leaders. It is used to threats of violence by those who have abandoned the façade of innocence. It is even used to acts of violence where civilian lives are taken carelessly or, arguably, deliberately in a military campaign. The brutal actions of Islamic State, formerly known as ISIS, however, are of a different order. There are no arguments. Far from apologising for their atrocities, IS militants have paraded them proudly on social media. A decapitated head is held up by the son of a jihadist, purportedly an Australian. A group of former soldiers are machine-gunned into a makeshift grave. Two young children are shown beheaded. A man is seen hanging as if crucified. Little wonder that a Vatican representative talked of "unspeakable criminal acts".

These acts, and their shameless broadcasting, have terrified Shias, Christians, and members of minority religions in Iraq, and horrified those outside the region. And this, of course, was one of the aims: fear of the Islamic State fighters has contributed to the melting away of the forces that oppose them. And now, not content with chasing Christians and Yarzidis into the barren mountains, Islamic State are reported to have shelled water sources to make life for the people they have displaced even more unbearable. It is impossible to imagine a religious justification for such behaviour.

The conduct of Islamic State militants makes the moral choices in Iraq very simple - simpler than they have been in the past. The country is in political turmoil after the discriminatory policies of Nouri Maliki, who this week was asked to give way as Prime Minister to Haider al-Abadi. But there is no question of a political accommodation with the Islamic State fighters: they simply have to be stopped and, if possible, brought to justice. What is holding the West back is largely logistical: the lesson learnt from its earlier encounters is that every military intervention needs an exit strategy. Given such a dysfunctional government, it is hard to see any effective hand-over if American and British troops are deployed to stop Islamic State's advance.

But there are no grounds for Western timidity. By its airdrops of food and water, the international community has acknowledged a humanitarian responsibility for the people driven from their towns and villages by Islamic State gunmen. US airstrikes have shown that there is no longer a clear line between aiding the oppressed and restraining their oppressors. But if the West refrains from being involved - one general called the Government of the UK "commitment-phobic" - there will soon be no indigenous allies. This is not an obscure war in central Africa. Iraq is familiar territory. It is well within the capabilities of the West to effect a significant improvement in the lives of a people for whom it remains responsible.