What Happened
The odds of a U.S. military response either directly against Iran or against the militias in Iraq backed by Tehran have risen following a Dec. 27 rocket attack on a military base in Kirkuk that left a U.S. contractor dead and four U.S. military personnel injured. At least 30 rockets struck the K-1 base in northern Iraq that houses both U.S. and Iraqi military forces. Both Islamic State militants and Iranian-backed militias known as Popular Mobilization Units (PMUs) have been active in the area, and both have conducted attacks around the base. But the scale of the Dec. 27 attack and the types of rockets launched make PMUs a more likely suspect than the Islamic State in this assault. In fact, U.S. officials have singled out Iranian-backed Kataib Hezbollah as a possible perpetrator.
Why It Matters
In mid-December, U.S. Secretary of State Mike Pompeo reiterated that any Iranian attack harming U.S. personnel would draw a "decisive U.S. response." That was just the latest in a drumbeat of similar warnings he has issued to Iraqi and Iranian leaders since June. Other U.S. officials have publicly and privately referred to any U.S. casualty as a result of Iranian action as a "red line." Any direct U.S. military response against Iran or its proxies, whether in the wake of this incident or others, will substantially increase the possibility of Iranian retaliation against U.S. interests that could spark a wider conflict. If a U.S. response remains limited to the PMUs in Iraq, it could set off a relatively contained exchange between the U.S. and Iranian proxies in the country. However, a direct U.S. military response against Iran would open the door for wider escalation, which in the extreme could draw in other U.S. allies, including Israel and Saudi Arabia.
Thus far in 2019, the United States has avoided taking military action after other Iranian-linked attacks, and it's possible that Washington will continue to pursue this risk-averse strategy. Additionally, despite the U.S. suspicions, Islamic State involvement in the K-1 attack cannot be ruled out. If it is shown that militants were the culprits, the risk of a cycle of U.S.-Iranian retaliation and escalation can be avoided. But if PMU involvement were to be proven, and the United States does not respond, it could embolden Iran or its proxies to launch additional attacks against U.S. interests.
Background
Given the presence of some 5,000 U.S. military personnel and numerous militias linked to Iran in Iraq, there is ample opportunity for incidents there to kindle a wider conflict. At least 10 rocket attacks attributed to Iranian-backed forces have occurred near Iraqi bases housing U.S. forces over the past several months, but none resulted in U.S. casualties. The United States has not responded militarily to other Iranian-linked attacks in the Middle East, including ones that devastated Saudi oil facilities, targeted vessels in the Persian Gulf and downed a U.S. drone, because they did not cause U.S. casualties.
