Who are the pro-Iran militias targeting U.S. troops?

Iran backs Hamas, the militant group that rules Gaza, and Hezbollah, the militant group and political party based in Lebanon. Since the Oct. 7 cross-border Hamas attack drew Israel’s retaliatory war in Gaza, attacks by militias linked to Iran have become a daily occurrence across the Middle East.

In Iraq and Syria, militia groups have targeted the U.S. military in 160 attacks since October. About 2,500 U.S. troops are based in Iraq and 900 in Syria that U.S. officials say are part of an ongoing operation to keep the Islamic State group from regaining a foothold in either and as a counterweight to Iran. Some high-profile attacks spanned:

» Syria: On Oct. 18, 20 troops suffered “minor injuries” after one-way drones targeted the al-Tanf Garrison in southeastern Syria, U.S. officials said.

» Iraq: In December, Iran-backed militias critically injured a U.S. service member in Iraq’s Irbil air base. In January, the Pentagon said that four American personnel were injured when militants launched ballistic missiles at Iraq’s Ain al-Asad Air Base, which houses U.S. troops.

» Jordan: Sunday’s deadly drone attack targeted the Tower 22 base in northeast Jordan near its borders with Syria and Iraq, which houses about 350 troops. The attack hit the facility’s living quarters and also injured at least 34 service members, a U.S. defense official said. It was not immediately clear where the attack was launched from, they said. The Islamic Resistance in Iraq, an umbrella group for Iran-linked militias, claimed responsibility for the attack. Iran has denied any involvement.

- Yemen: The Houthis, an Iran-allied rebel group, started targeting ships in the Red Sea in November, including U.S. warships. The group’s continued attacks against one of the world’s busiest waterways have taken an outsize bite of global shipping.

Who are the pro-Iran militias targeting U.S. troops?

The Islamic Resistance in Iraq claimed responsibility for Sunday’s attack in Jordan, days after claiming the attack on Iraq’s Ain al-Asad Air Base.

This is a loosely formed group of militias that include Kataib Hezbollah and Harakat Hezbollah al-Nujaba. The Islamic Resistance in Iraq “is an umbrella organization of armed groups and militias that are aligned with Iran and that see themselves as vanguards fighting against Israel and American imperialism in the region,” Renad Mansour, a senior research fellow at London’s Chatham House think tank, said in a telephone interview Monday.

Iran has cultivated the militias, which benefit militarily from Tehran’s support, for years as a way of securing its influence across the region. “They have their own interests, but their interests often converge on Iran’s interest - which is to maintain their power across the region,” he said.

In the two decades since the United States invaded Iraq in 2003 and toppled Saddam Hussein’s regime, Iran has used this group of militias to expand its influence in the country, Mansour said. After the withdrawal of U.S. troops in 2011, the militias increasingly took the lead in Iraq’s fight against the Islamic State, which had taken over a third of the country by 2014 as government security forces collapsed. Now, Mansour said, the militias continue to control the large swaths of territory once held by the Islamic State across the border area straddling Syria and Iraq — where Sunday’s drone attack was probably launched.

How has the United States responded?

The Biden administration is looking to prevent violence in Gaza from spilling out across the region, and responded cautiously to the attacks. In October, President Biden said he warned Iran’s supreme leader, Ayatollah Ali Khamenei, that “we will respond” if Tehran continued to “move against” U.S. forces in the region.

Since, the administration has announced regular “precision self-defense” retaliatory airstrikes in Iraq and Syria. Little is known about the attacks in Syria, whose government rarely announces death tolls or the extent of the damage from such strikes. The White House is facing pressure from some critics who say the targeted retaliatory attacks are not enough and want the administration to take more significant military action. (It is not the only flash point on the Biden administration’s radar. In Yemen, a U.S.-led coalition has repeatedly struck Houthi militant infrastructure. In the Red Sea, the United States formed a multinational naval coalition to protect commercial shipping.)

On Sunday, Biden vowed to respond to the latest attack in Jordan, which he blamed on “radical Iran-backed militant groups.” He continued: “We will hold all those responsible to account at a time and in a manner our choosing.”

For its part, a senior official in the Islamic Resistance in Iraq said that the group would continue with its “escalations” as long as the United States maintains its support for Israel. “All U.S. interests in the region are legitimate targets,” they said, speaking on the condition of anonymity in line with rules set by the group. “We don’t care about U.S. threats to respond. … Martyrdom is our prize.”

What does Iran say?

Tehran had no connection and had nothing to do with the attack in Jordan, Iran’s permanent mission to the United Nations told the country’s official Islamic Republic News Agency.

A major clash between the United States and Iran would have huge implications, and the rhetoric from both sides has been relatively restrained. As Iranian-backed groups continue to attack troops and Western interests, “the million-dollar question” is how directly is Iran involved, Hamidreza Azizi, a visiting fellow at the German Institute for International and Security Affairs, previously said.

Washington Post correspondents, reporters and bureau chiefs Liz Sly, Louisa Loveluck, Kareem Fahim, Mustafa Salim, Suzan Haidamous, Alex Horton, Toluse Olorunnipa, Dan Lamothe and Missy Ryan contributed to this report.