Updated February 2, 2024 at 6:03 PM ET

The U.S. military has mounted a series of air and missile strikes against Iranian proxies in Iraq and Syria, NPR has confirmed, in retaliation for a suicide drone strike that killed three American soldiers on Sunday at a remote base in Jordan.

The airstrikes, which used more than 125 precision munitions, came at 4 p.m. ET Friday and struck more than 85 targets, U.S. Central Command said in a statement.

"The facilities that were struck included command and control operations, centers, intelligence centers, rockets, and missiles, and unmanned aired vehicle storages, and logistics and munition supply chain facilities of militia groups and their IRGC sponsors who facilitated attacks against U.S. and Coalition forces," CENTCOM said, referring to Iran's Revolutionary Guard Corps.

John Kirby, a spokesman for the National Security Council, told reporters Friday that in all seven facilities used by IRGC and its proxies were hit — three in Iraq and four in Syria. The strikes, he said, occurred over 30 minutes. The Iraqi government was informed beforehand, he said.

The targets were chosen to avoid civilian casualties, he said.

The U.S. strikes are far more extensive and deadly than those that have been launched since last October, when the Israeli-Gaza war began and pro-Iranian groups like the Houthis in Yemen and militias in Iraq started an uptick of attacks.

That's because the Jan. 28 attacks on a U.S. support base in Jordan was the highest death toll of troops in the Middle East in at least a decade. Three Army Reserve soldiers from Georgia were killed when an attack drone slammed into their sleeping quarters. Another three dozen were wounded, a handful seriously. President Biden traveled Friday to Dover Air Force Base in Delaware, where the bodies of the three soldiers returned in flag-draped silver cases.

In a statement after the strikes, Biden said: "Our response began today. It will continue at times and places of our choosing. The United States does not seek conflict in the Middle East or anywhere else in the world. But let all those who might seek to do us harm know this: If you harm an American, we will respond."

The U.S. attacks were telegraphed for days. Biden told reporters outside the White House earlier this week he had decided on a response. And lawmakers were told by senior administration officials that the president wanted military options "a level or two up" from the "whack-a-mole strikes we had been doing on (militia) storage and launch sites."

National Security spokesman John Kirby told reporters that the initial American strikes were likely just the beginning.

"It's very possible you will see a tiered approach here," Kirby said, "not just a single action, but potentially multiple actions over a period of time."

Republican lawmakers have pressed Biden to strike Iran itself because it has trained and supported the militia groups that have targeted American troops, while Democrats — and Biden himself — have been more reluctant about targeting Iran and widening the Israeli-Gaza war into a regional conflict.

Calls for a more aggressive military response

Retired Adm. James Stavridis wrote in Bloomberg that the deaths of the three American soldiers required Biden to respond with a "new level of force." That should include "continuous strikes against proxy targets in Syria and Yemen," while working with Iraq "to expand strikes to their country."

Stavridis also said the U.S. should be "prepping for a significant cyber attack" on Iran, including severing its ties to its proxy forces, penetrating its oil and gas infrastructure and reducing its armament production.

Brad Bowman, a Middle East analyst at the Foundation for Defense of Democracies, said it was essential for the U.S. to "hit back hard to restore tattered American deterrence." But he worried whether the U.S. troops in the region had enough defenses to "prepare for the inevitable counterpunch that will come after U.S. airstrikes." Pentagon officials have been reluctant to talk openly about the possibility of any increased American defenses in the region.

Charles Lister, an analyst at the Middle East Institute, said it appeared the Biden administration was moving from strikes to deter Iranian-backed militias and Iranian units themselves to efforts that would degrade their power. It all seems like a "campaign," he said, rather than a "single round of strikes."

Lister said that beyond the Iranian-backed militias, there are "plenty of target options" including bases in Syria where the Iranian Revolutionary Guard is located to Iranian naval assets, such as the MV Behshad, an Iranian cargo ship suspected of being a spy platform helping Houthi militants target shipping in the Red Sea.

"But that'd feed into the 'regional war' language that the Biden administration remains keen to avoid," Lister said.

Ongoing attacks

The Iranian-backed militias have mounted more than 165 drone, missile and rocket attacks on U.S. troops in Iraq and Syria since last fall. Most led to minor injuries, though one service member took some shrapnel to the head and was sent back to the U.S. for further treatment.

The U.S. has repeatedly hit targets in Syria and Iraq, though the militias continue to strike at American targets — part of Iran's strategy of pushing the U.S. out of the region.

Last October, the U.S. responded with airstrikes by bombing weapons and storage facilities in Syria with F-16 jets.

A few weeks later, the U.S. hit more targets in Syria, including those used by the Iranian Revolutionary Guard and its allies. According to the Syrian Observatory for Human Rights, nine workers in the facilities were killed in the strikes.

On Nov. 21, 2023, U.S. Air Force AC-130 gunship retaliated against the Iranian-backed militia Kataib Hezbollah, striking a vehicle near Abu Ghraib, Iraq, in response to an attack the day before. U.S. forces at al Asad Airbase west of Baghdad. According to U.S. assessments, several Iran-backed fighters were killed in the strike. The next day, U.S. fighter jets conducted airstrikes on Iraqi Kataib Hezbollah facilities, killing over eight fighters.

And there were two strikes in December by the U.S. in Iraq, hitting militants near Kirkuk, and killing five as they attempted to fire on U.S. forces. Another one hit a militia base in Hillah, killing one militant and wounding 20 others.

What is the "axis of resistance" and Kataib Hezbollah?

The so-called "axis of resistance" is a network of Iran proxy groups across the region. It includes Hezbollah in Lebanon, a coalition of militias in Iraq that go by the name the Islamic Resistance in Iraq, Ansar Allah — Houthi forces — in Yemen and Iran-linked groups in eastern Syria.

Kataib Hezbollah (which means Party of God Brigades) is not a subsidiary of the Hezbollah militia in Lebanon, but both have close links to Iran. KH is considered the most powerful of the Iran-backed militias in the Iraq-based resistance, which has claimed about 160 attacks on U.S. forces since the beginning of the Gaza war.

It was one of the many Shiite militias that fought ISIS starting in 2014 and along with others, was incorporated into Iraq's official security forces in 2019. Before the Gaza war, the group was known for attacks on the U.S. military which it considers to be occupying forces in Iraq, including using roadside bombs manufactured in Iran.

The U.S. killed the founder of KH, Abu Mahdi al-Muhandis, in the same drone strike four years ago in Baghdad that killed Iranian general Quassem Soleimani. Muhandis was also a senior Iraqi government security official and that killing dramatically raised tension in Iraq with U.S. forces.

On Jan. 24, the U.S. struck three bases of Kataib Hezbollah in retaliation for strikes that included an attack on the Ain al-Asad base in Western al-Anbar. U.S. officials have said that the attack on a remote U.S. base in Jordan that killed three service members last week had the fingerprints of KH.

Iran does not exert complete command over the proxies and most, like Lebanon's Hezbollah, have their own domestic agendas.

An evolving mission for U.S. troops

U.S. troops left Iraq in 2011 but returned in 2014 to fight ISIS alongside Iraqi forces. And now there are 2,550 — not in combat, but assisting Iraqis in going after the remnants of ISIS. Now, many of the U.S. troops are stationed in northern Iraq in Erbil, and those troops also support the anti-ISIS fight next door in Syria.

The U.S. also provides hundreds of millions of dollars to Iraq in aid, government development, humanitarian assistance, demining efforts and military sales --more than $16 billion, covering everything from F-16 aircraft to helicopters and radar and small arms. In addition, the U.S. has provided Iraq with excess defense equipment over the recent years — 300 large armored vehicles, Humvees, helicopters, body armor — all of which contributed to the ISIS fight.

Still, the U.S. airstrikes have created a political problem for the Iraqi government, some of whose parliamentarians have ties to Iran and want to see U.S. forces withdraw from the country. And the latest strikes will certainly add to that headache.

Prime Minister Mohammed Shia al-Sudani came to power because Iran, and the militias it supports, backed him. So in between anger over the U.S. role in supplying Israel with weapons for the war in Gaza, and anger over U.S. breaches of Iraqi sovereignty, he faces intense pressure about the future of U.S. forces in Iraq.

Now the U.S. and Iraq are talking about an "evolving" mission for U.S. troops, and there seems to be a disconnect.

An adviser to the Iraqi prime minister said the aim is to come up with what he called a specific and clear timetable for the gradual reduction of the U.S.-led coalition troops in Iraq and to an end to the U.S.-led anti-ISIS mission. The government spokesman, Bassem al-Awadi, told Iraqi state TV viewers that Sudani had repeatedly made clear that Iraq's stability required ending the U.S. military presence in Iraq.

"He literally said that ending the mission of the international coalition in Iraq is necessary for Iraq," said al-Awadi. "He used the term necessity. And I assure you that when the prime minister uses a term, he means it."

That's not how the U.S. sees it. In a background call recently senior Pentagon and State Department officials said the talks are not about a withdrawal of U.S. troops. They said it's about shaping the future of the U.S. military presence. That presence will be determined, they say, by the strength of ISIS and the capability of Iraqi forces.

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