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Soon after this announcement, two US allies came to blows. The city of Kirkuk had, until yesterday, been occupied by the Iraqi Kurdish Regional Government (KRG). For many Kurds, Kirkuk holds special significance: for some, it is nothing less than their "Jerusalem".

The KRG had taken charge of the city in the course of the fight against the Islamic State group (IS). And while it is not part of the official Kurdish autonomous zone, it was embedded within Iraqi Kurdish plans for self-defence, and occupies a significant position in the proposed independent state which the vast majority of Iraqi Kurds voted in favour of creating last month.

Residents of Kirkuk did not have a vote in the independence referendum. But its result has affected them directly and rapidly nonetheless.

With no warning, the Iraqi state's security forces and the militias of the Popular Mobilisation Forces (PMF), some of which are Iranian proxies, moved into Kirkuk, ostensibly to take possession of federal institutions and bases in the city.

Though what has happened in Kirkuk has primarily been orchestrated by the Iraqi state, it could not have happened without the approval of Soleimani

Not only did this turn into more general occupation - with the appointment of a new governor by Iraq's federal authorities - but it also led to violent clashes between various Kurdish groups and the factions representing the Iraqi state.

To return to Trump's intervention last week, some of those latter factions, are not in reality loyal to the Iraqi government. Instead, they are Iranian "boots on the ground", superintended by major-general Qassem Soleimani of the IRGC-Quds Force.

The foreign operations it handles include supporting the mass-murdering regime of Bashar al-Assad in Syria financially, as well as marshalling a sectarian "Shia jihad" of foreign fighters to defend it. In Iraq, Iranian-backed Shia militias have committed terrible atrocities for over a decade, but these have become institutionalised within the PMF component of the Iraqi campaign against IS. The Americans know this, and also see it in Iraq, Syria, Lebanon, Yemen and elsewhere.

In denouncing the Iranian regime, and designating the IRGC under the terrorism authority in the US, Trump has recognised the state terrorism of Iran, and the means by which much of it is carried out. This looks to be the beginning of a stronger American response to Iran's overseas manoeuvres.

Though what has happened in Kirkuk has primarily been orchestrated by the Iraqi state, it could not have happened without the approval of Soleimani. He and Iran were more than willing to co-ordinate with Iraq to attack the KRG.

Many details of the attack are still unconfirmed. Dexter Filkins in The New Yorker that Soleimani, through a  "mix of threats and inducements, including money and access to oil-smuggling routes", persuaded fighters affiliated with the Patriotic Union of Kurdistan (PUK) - one of the KRG's major parties - to abandon their posts ahead of the Iraqi advance.

If true, this is a remarkable demonstration of Iran's strong and growing power within Iraqi circles. It demonstrates the extent to which the IRGC, a designated terrorist organisation, is openly working with one American ally (and recipient of American weapons) in the Iraqi state, to attack and victimise another: The KRG.

The Americans are in a very difficult position. US diplomacy, which first sought to dissuade Iraq's Kurds from holding a referendum and then attempted to minimise the problems this referendum could cause between Iraq and its Kurdish population, has signally failed in the recent past.

American policy in the Trump era is profoundly inconsistent. The president frequently fails to sustain and structure his responses to foreign events

Brett McGurk, the US special envoy to the anti-IS coalition, has been consistent in tone but has achieved little. His plaintive suggestion that all sides cease fighting and continue to focus on what remains of the caliphate claimed by IS is likely in vain. And it has been retrospectively proven useless by the force of what has happened in the last few days.

President Trump has a very difficult job to do.

The logic of Trump's position dictates that he must respond very strongly to what has happened. Iranian–Iraqi aggression against the KRG is unacceptable, no matter the American opposition to the Kurdish referendum. That the Iraqi advance included militias organised by a newly-designated terrorist organisation, and was possibly paved by IRGC subterfuge, should focus his mind.

Logically, Trump must respond.

Kyle Orton, a research fellow at the Henry Jackson Society's Centre for Radicalisation and Terror, explains that "The operation against Kirkuk is the first major test of the US designation of the IRGC as a terrorist organisation."

"Not only are IRGC agents and proxies infiltrated throughout the system - the Badr Corps controls the Interior Ministry and the Federal Police, for example - but militias openly loyal to the IRGC and themselves designated terrorist organisations like Kataib Hizballah are on the frontlines," Orton said.

This gap between presentation and reality, implication and policy, has never been more apparent than in relation to Kirkuk

"If the US cannot prevent or reverse an IRGC-led offensive against one of its closest regional allies, then the designation looks meaningless."

But American policy in the Trump era is profoundly inconsistent. The president frequently fails to sustain and structure his responses to foreign events. He acts once, often with seeming decisiveness, and then fails to do anything more drawn-out and more ultimately useful.

This gap between presentation and reality, implication and policy, has never been more apparent than in relation to Kirkuk.

The Americans first put out a statement in which they suggested that clashes between Kurds and Iraqi state forces were a "mistake", and then simply attempted to talk each participant out of open conflict, sparing American blushes.

In the last few days, the US has allowed the proxy forces of the adversary, working in concert with the Iraqi state, openly to attack an American ally. It will not be forgotten. And the repercussions could be very serious indeed.

James Snell is a writer whose work has appeared in numerous international publications including The Telegraph, Prospect, National Review, NOW News, Middle East Eye and History Today.

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Opinions expressed in this article remain those of the author and do not necessarily represent those of °®Âþµº, its editorial board or staff.