Although most Americans don’t know it, and certainly haven’t endorsed it, the United States is back at war in the Middle East. Its latest, and most hideous, antagonist is the monstrous aberration that calls itself the Islamic State. And no matter what the present intentions, there seems no way this conflict in Iraq and Syria can fail to metastasise.

For the past year or so, the Islamic State has been on a roll and a rampage, sweeping across huge sections of northern Syria and, more recently, western Iraq. Nobody did much of anything to stop them, and American intelligence officials have acknowledged that they were taken aback by the lightning speed of the Islamic State’s advance.

Last week, the Islamic State presented the Obama administration with an impossible two-fold conundrum. On the one hand, it had driven thousands of Yazidi religious minorities onto an isolated mountaintop where they faced certain death if not relieved. On the other hand, it was threatening to advance towards the Kurdistan Regional Government capital of Ebril where the United States has a consulate and numerous offices. From both a humanitarian and a practical point of view, the president had no choice.

Barack Obama interrupted his holiday to inform Americans he had authorised the first US air strikes in Iraq in years, to address both problems. In the event, they turned out to also be acting in direct support of a counter- offensive by elite Iraqi troops and Kurdish fighters aimed at seizing control of the strategically vital Mosul Dam from Islamic State terrorists. The US has now flown dozens of sorties and Mr Obama says the retaking of the dam was successful.

No one was quite sure where that left matters, but subsequent events seem to have answered all questions. First, the use of American air power against the terrorists continues. Second, Islamic State fanatics, apparently operating in their Syrian stronghold, beheaded a captured American journalist, James Foley, on video. They openly said it was retaliation for the American air strikes and threatened to murder more captured Americans, at least one of whom was displayed on camera. They also threatened to drown westerners with blood and issued similar blood-curdling threats.

It is often said that the Islamic State is a crafty and calculating organisation. There is no sign of that here. Had they been intelligent, the fanatics would have allowed the American system to produce its own push back against Mr Obama’s initiative to challenge them. There was, and still is, a good deal of scepticism in Congress and among the public against any further American military engagement in Iraq or anywhere else in the Middle East.

But with the Foley murder, and particularly the gruesome video and photographs they distributed, the Islamic State poked the lion in the eye.

The Islamic State seemed to be sending a series of calculated messages with the video: if you attack us, we will murder you. You didn’t pay the $132 million ransom for this guy we demanded, so it’s only natural that we kill him, especially if you are attacking us. I’m speaking loudly and clearly in a distinctly British accent, so understand that we can very easily come to you. And don’t forget that our main base is in Syria, and you’re not coming here.

If I had wanted to give the followers of the new “caliphate” terrible advice, I couldn’t have come up with anything more foolish than that. It strongly suggests that the Islamic State just doesn’t know what it’s dealing with in the United States. Such threats will only provoke Americans, particularly the implicit threat to carry out terrorist attacks in the West and, especially, in the United States itself.

Given the way that the stakes have been raised by both sides very quickly, it’s hard to imagine that they will not continue to escalate. It’s true that a month ago, few Americans would have been ready to embrace the current conflict, and many still are not. But they’re getting there quickly. The threat, and even more, the reality of terrorist attacks against Americans will only increase that determination.

This is, after all, very much already Mr Obama’s war. In campaigning for the nomination and the presidency, he emphasised his opposition to the war in Iraq and the nation-building programme in Afghanistan, contrasting them with the war on terrorism that he strongly supported. And he’s proven that during his presidency with covert actions and drone attacks that have been controversial, but gained such scalps as Osama bin Laden.

The Islamic State is, US administration officials now admit, a terrorist threat on a magnitude not seen before. It seems a combination of the worst of the Taliban and the worst of Al Qaeda rolled together, in the heart of the Middle East and with a growing contingent of international fanatical volunteers, including many westerners.

How, precisely, to roll them back without relying on sectarian Shiite forces in Iraq or, even worse, aiding Syrian dictator Bashar Al Assad, are conundrums inherent in the mission. But America’s new war in the Middle East is a necessary, and not optional or avoidable, one.