“At President Biden’s direction, U.S. military forces earlier this evening conducted airstrikes against infrastructure utilized by Iranian-backed militant groups in eastern Syria,” Pentagon spokesperson John Kirby said Thursday in a statement sent to Newsweek. “These strikes were authorized in response to recent attacks against American and Coalition personnel in Iraq, and to ongoing threats to those personnel.”

An unclaimed rocket barrage killed a contractor and injured another, along with a U.S. soldier in Iraq’s northern city of Erbil, the capital of the semi-autonomous Kurdistan region. Both the U.S. and Iraqi governments condemned the attacks, and Washington offered assistance in the investigation, hinting at Tehran’s involvement in the days since.

Kirby identified the targets Thursday as positions held by two Iran-backed militias located along the Syria-Iraq border.

“Specifically, the strikes destroyed multiple facilities located at a border control point used by a number of Iranian-backed militant groups, including Kait’ib Hezbollah (KH) and Kait’ib Sayyid al-Shuhada (KSS),” he said. “This proportionate military response was conducted together with diplomatic measures, including consultation with Coalition partners.”

He also issued a direct message from the president.

“The operation sends an unambiguous message: President Biden will act to protect American and Coalition personnel,” Kirby said. “At the same time, we have acted in a deliberate manner that aims to de-escalate the overall situation in both eastern Syria and Iraq.”

Defense Secretary Lloyd Austin later confirmed the news when questioned by reporters, telling them the operation was a success.

“We’re confident that the target was being used by the same Shia militia that we’re confident in the target we went after, we know what we hit,” Austin said. “We’re confident that the target was being used by the same Shia militia that conducted the strikes.”

He called the Biden administration’s approach “very deliberative” and credited the investigation conducted by Iraqi authorities in the wake of the Erbil attack with helping to pinpoint the alleged perpetrators.

Militias supportive of Iran operate openly in both Iraq and Syria as part of a mission to battle jihadi elements following the rise of the Islamic State militant group (ISIS), but the U.S. and regional allies such as Israel consider them to be a destabilizing factor.

The U.S.-led coalition operates a separate counterterrorism mission in both countries, but does so in Syria in coordination with a largely Kurdish force known as the Syrian Democratic Forces, not the central government in Damascus. The Syrian government, for its part, has received support from both Russia and Iran in its near-decade-long war against jihadis and various rebel groups, some of which were once backed by the U.S. under former President Barack Obama, whom Biden served as vice president.

While Washington and Tehran both oppose ISIS, the two longtime rivals have increasingly been at odds since former President Donald Trump’s unilateral exit from a nuclear deal forged under Obama in 2015.

The Trump administration enacted harsh sanctions on Iran and instituted a campaign of “maximum pressure” that was followed by increased incidents between U.S. troops and Iraqi militias aligned with Iran. During this period, Trump too hit Kataib Hezbollah positions in eastern Syria, and also bombed the group in its native Iraq, where he ordered the assassination of top Iranian military leader Revolutionary Guard Quds Force commander Major General Qassem Soleimani as well.

Since coming to office, Biden has signaled a willingness to rejoin the nuclear agreement—officially known as the Joint Comprehensive Plan of Action (JCPOA)—but has called on Iran to first reinstitute suspended restrictions on its nuclear program, a move Iranian officials argue would come immediately after a U.S. reentry.

The Biden administration has since signaled a willingness to join talks overseen by the European Union alongside nuclear deal parties China, France, Iran, Germany, Russia and the United Kingdom but Iranian officials have noted that the U.S. was no longer a part of what was known as the P5+1 format.

On Wednesday, State Department spokesperson Ned Price warned that “our patience is not unlimited” in reference to Iran, and expanded upon these remarks on Thursday, prior to the U.S. strikes in Syria.

“Tehran is under significant pressure, sanctions pressure from the United States and well beyond that,” Price told reporters. “I made the point yesterday, the point you refer to, precisely because for us this is an urgent challenge. And again, to be clear, I’m not speaking of the urgency of rejoining the Joint Comprehensive Plan of Action. For us, that is a tactic. It is a tactic to achieve our strategic aim, and that strategic aim is to ensure that Iran can never be in a position to acquire a nuclear weapon.”

Iranian Supreme Leader Ayatollah Ali Khamenei has issued a religious ruling against nuclear weapons, and Iranian officials argue the Islamic Republic has never sought one.

As the U.S.-Iran saga plays out, Syria continues to be the target of a number of foreign aerial attacks, including from Israel, which also targets sites suspected of being linked to Iran. The Russian and Syrian air forces target insurgent groups located in the northern region border Turkey, which sponsors opposition forces that also rival U.S.-backed Syrian Democratic Forces.

With Washington’s diplomatic role uncertain, Moscow, Tehran and Ankara have established a trilateral peace process in hopes of solving the conflict set to mark its 10th anniversary next month.