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The killing by the US of Qassem Soleimani, the head of Iran's Islamic Revolutionary Guards Corps Quds Force, marks a dramatic turning point.

Soleimani had been in Washington's crosshairs for many years, and successive US presidents could likely have ordered his assassination in the past. That they chose not to do so suggests that they worried the costs would outweigh the benefits.

With his decision, President Donald Trump is making clear that he abides by a different calculus: that, given the vast power imbalance, Iran has far more to fear from war than does the US. The strike that killed the Iranian general along with others - notably Abu Mahdi al-Muhandis, a senior commander of the pro-Iranian Iraqi Shia militia - was, in accordance with this view, meant as a deterrent to further Iranian attacks.

It is almost certain to be anything but. Iran may fear US retaliation, but it fears projecting that fear even more. From its perspective, it cannot allow what it views as a declaration of war to remain unanswered.

It will respond and now must decide whether its reaction will be direct or through the array of proxies and allied forces Soleimani helped build; immediate or deferred; in Iraq or elsewhere - in the Gulf, Syria or beyond.

The US presence in Iraq, already shaky after the 29 December strike that killed two dozen members of a pro-Iranian Iraqi militia, is now hanging by a thread; the Trump administration may decide to depart preemptively rather than be forced to leave on Baghdad's orders.

The truce in Yemen between Saudi Arabia and Iran-backed Houthi fighters also is in greater jeopardy. Watch in particular for Iran's announcement of its next steps on the nuclear front, taken in response to Washington's violation of the 2015 deal. A serious step on 6 January had been predicted; in all likelihood, it just got far more serious.

Read more: Who was Qassem Soleimani, the powerful commander of Iran's elite Quds force?

The US-Iranian game has changed. Their rivalry for the most part played out as an attritional standoff: Washington laying siege to Iran's economy in hopes that financial duress would lead either to its government's capitulation to US demands or to its ouster; and Tehran responding with actions that maintained a veneer of plausible deniability. Targeting Soleimani is liable to mark a shift from attrition toward open confrontation.

In short, a US president who repeatedly claimed that he does not wish to drag the country into another Middle East war has just brought that war one step closer. And a US administration that argues it killed the Iranian general in order to avert further attacks just made those attacks more likely. Iran will retaliate; the US will avenge the retaliation; and many across the region will pay the price.

Crisis Group is in the business of policy recommendations aimed at averting conflict. It is also in the business of realism. Some kind of conflict is now all but guaranteed, facilitated no doubt by a series of Iranian actions of which Soleimani was a mastermind, but rooted in President Trump's ill-advised and reckless decision to exit the nuclear deal and embark on a policy of "maximum pressure" that led, almost inexorably and certainly predictably, to today's crisis.

Iran may fear US retaliation, but it fears projecting that fear even more

The outcome is all the more tragic because the contours of a solution have been apparent for months: a tactical détente whereby Iran fully restores its compliance with the nuclear deal, and ends its regional provocations, in return for a reprieve from the crushing impact of US sanctions.

One can only hope that, with encouragement and pressure from the two sides' respective allies, this perilous tit-for-tat will be relatively contained and of relatively short duration. That, after a few rounds of attack and counter-attack, Washington's desire to avoid getting sucked into another Middle East war and Tehran's interest in averting devastating US strikes, will drive both toward de-escalation.

One can only hope. The alternative is too horrific to contemplate.

Robert Malley is president and CEO of International Crisis Group. 

 is a transnational non-profit, non-governmental organisation that carries out field research on violent conflict and advances policies to prevent, mitigate or resolve conflict.

Republished, with permission, from the .

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