Proxy forces armed by Iran could take part in retaliation against Israel over Hamas leader's killing

DUBAI, United Arab Emirates -- As Iran threatens to respond to the suspected Israeli assassination of Hamas leader Ismail Haniyeh in Tehran, the regional militias that the Islamic Republic has armed for decades could play a role in any attack.

Here's a look at Iran's history of arming militias, its allies in the region and what part they could play.

Why has Iran armed proxy forces?

Iran's policy of arming militias took root in the aftermath of the 1979 Islamic Revolution. Before it, the U.S. provided major weapon systems including F-14 Tomcat fighter jets to the government of Shah Mohammad Reza Pahlavi. After the revolution and the U.S. Embassy hostage crisis, those shipments and needed maintenance programs stopped. Iran's eight-year war with Iraq in the 1980s destroyed much of its arsenal. International sanctions on Iran, including over its nuclear program, also have kept it from receiving new arms while Israel and Gulf Arab states allied with the U.S. have received advanced weapons. While developing its own missile program, Iran can't match those sophisticated weapons. It relies on militias as an asymmetric threat to squeeze both Israel and the United States.

Who are Iran's regional allies?

Iran's arming began in earnest in the 1980s with Shiite forces in Lebanon fighting against Israel. They became the Hezbollah militia. The arming expanded with the 2003 U.S.-led invasion of Iraq that toppled dictator Saddam Hussein, a longtime foe of Tehran. Iran strongly backed Syrian President Bashar Assad in his country's long war. And Iran has continued when the opportunity has arisen, even arming Sunni militants while viewing itself as the world's defender of Shiite Muslims. Those relationships are managed by Iran's paramilitary Revolutionary Guard, one of the most powerful armed groups in the Middle East.

The militias in Iran's self-described “Axis of Resistance” include these:

Iraqi militias

In Iraq, Iran supported a slew of forces that mobilized in 2014 to battle the Islamic State group. Those state-sanctioned, mainly Shiite militias, known as the Popular Mobilization Forces, have grown into a powerful political faction, armed with rockets, drones and other weaponry. The International Institute for Strategic Studies puts their strength at some 180,000 fighters. Other smaller or little-known militant groups have emerged and claimed attacks against U.S. forces as well amid this Israel-Hamas war. Iran-backed armed groups attacked U.S. personnel in Iraq more than 60 times between October and Feb. 4, according to the Congressional Research Service. The deadliest came on Jan. 28, when the U.S. said a drone launched by Iranian-backed Iraqi militias hit a facility known as Tower 22 in Jordan on the Syrian border, killing three American troops and wounding dozens of others. In response, U.S. airstrikes hit more than 85 targets at seven locations, including command and control headquarters, intelligence centers, rockets and missiles, drone and ammunition storage sites and other facilities connected to the militias or the Guard's expeditionary Quds Force.

Lebanon's Hezbollah

Hezbollah formed in 1982 amid Israel's occupation of southern Lebanon. Israel remains highly wary of Hezbollah, particularly over the vast missile arsenal it is believed to possess and its battle-hardened forces who also supported Assad in Syria. While Israel has sophisticated missile defenses including its Iron Dome system, a mass barrage of fire from Hezbollah and others at the same time could overwhelm the country. Estimates suggest Hezbollah has an arsenal of 150,000 rockets and missiles, including precision-guided missiles. The militia also has been blamed for suicide bombings in the past, including a 1983 bombing in Beirut that killed 241 American servicemen, though the group maintains it wasn't behind the attack. Hezbollah also has drones and surface-to-air missile systems. Hezbollah's forces number as many as 25,000 full-time fighters, with additional tens of thousands in reserves, according to an Israeli military assessment. Hezbollah's leader Hassan Nasrallah in 2021 said the group had 100,000 trained fighters.

Palestinian militant forces

Despite being Sunni, both the Palestinian militant group Hamas and Islamic Jihad have received weaponry and other materiel from Iran. The groups, however, have been struck hard by Israel since the Oct. 7 Hamas attack that began the war, which saw militants kill 1,200 people and take 250 others hostage. Israel's war on Hamas in the Gaza Strip since has killed at least 39,580 Palestinians, according to Gaza’s Health Ministry, which does not distinguish between civilians and combatants in its count. Israel's military says it has killed roughly 15,000 militants in the war.

Yemen's Houthi rebels

The Houthis have held Yemen’s capital, Sanaa, since 2014 as part of that country’s ruinous war. They follow the Shiite Zaydi faith, a branch of Shiite Islam that is almost exclusively found in Yemen. While broadly an insurgent force, the group with Iran's support is now able to launch drone and missile attacks that have drastically disrupted shipping in the Red Sea corridor and now even reach Israel. The U.S. Navy's efforts at stopping the shipping attacks have led to the most intense continuous combat its sailors have faced since World War II, but has yet to end the assaults. The amount of direct command Iran wields over the Houthis, however, remains a matter of debate among experts. The Houthis' attacks have raised their international profile while cracking down on dissent at home. The rebels claim they've recruited 200,000 additional fighters since launching their attacks. The rebels and their allies have a fighting force of some 20,000 fighters, according to the International Institute for Strategic Studies.

What could any retaliatory strike look like?

In April following an Israeli attack on the Iranian Embassy compound in Syria, Iran launched 170 bomb-carrying drones, more than 30 cruise missiles and more than 120 ballistic missiles toward Israel. Israel, the U.S. and other nations shot down many of the projectiles, some of which came from Yemen. Iran could launch a similar assault, but this time Hezbollah may get involved as the militia seeks revenge for the Israeli strike last week killing senior commander Fouad Shukur. Such an assault could strain Israeli air defenses, meaning more missile strikes raising the risk of casualties — and of a further escalation experts fear could lead to a wider regional war.