On Sunday, Jan. 28, another Iran-backed militant group known as Islamic Resistance in Iraq launched a drone attack on a small outpost in the country of Jordan, killing three U.S. Army soldiers and injuring more than 30 service members. “We shall respond,” President Joe Biden said on Sunday. Over the past four months, Iran-backed militant groups have increasingly attacked both civilians and military forces throughout the Middle East. The increase in violence began when Hamas launched a surprise attack on Israel, killing 1,400 people in what has been referred to as Israel’s 9/11.

Since November, another group, known as Houthis, have launched what the State Department describes as “unprecedented attacks” against international maritime vessels in the Red Sea and Gulf of Aden, as well as military forces positioned in the Arabian Peninsula to defend the safety and security of commercial shipping. These attacks against international shipping, says the State Department, have “endangered mariners, disrupted the free flow of commerce, and interfered with navigational rights and freedoms.”

Here is what you should know about the Iran-backed terrorist organizations responsible for numerous recent attacks in the Middle East.

How is Iran involved in the recent attacks?

Although Iran denies being involved in the attack on the U.S. outpost, the Shia-dominant nation has been accused of providing weapons and funding for several militant groups in the Middle East including the Houthis, the Islamic Resistance in Iraq, Hamas, and Hezbollah. Earlier this year, the Iranian foreign ministry spokesperson said that “Iran proudly announces that it supports Palestinian resistance movements for the liberation of their land.”

These militant groups are used by Iran to fight a proxy war against Iran’s enemies—namely Saudi Arabia, Israel, and the U.S. (In proxy wars, states arm and support actors in another country to achieve their broader geopolitical goals.) Iran-linked groups using drones packed with explosives have attacked U.S. troops in the Middle East more than 150 times since Hamas’s recent assault on Israel.

Who are the Houthis?

The Houthis—formally known as Ansar Allah or Ansarallah (Supporters of God)—originated in the early 1990s in northern Yemen. They began as a theological movement and took their name from their original leader, Hussein Badreddin al-Houthi, who served in the Yemeni parliament and advocated for the rights of the Zaidi Shia Muslim minority in Yemen. 

Zaydi Shiites are a minority within the Islamic world, with distinct beliefs from other Shiite groups (The two main branches of Islam are Sunni and Shia, which differ in their views on political succession and the authority of Muhammad’s descendants. Roughly 90% of the world’s Muslims are Sunni and 10% are Shia.) 

The Houthis follow a branch of Shia Islam known as Zaidism, but the Houthi movement represents a complex blend of religious, political, and tribal elements.They seek greater autonomy for their region and oppose what they perceive as Western influence and Sunni dominance, promoted by Saudi Arabia and other Gulf states. The Houthi emblem, which offers a general idea of the group’s views, is composed entirely of the following phrases: “God is great, Death to America, death to Israel, damnation to the Jews, victory to Islam.” 

The Houthis have said the reason they were targeting ships that are Israeli owned, flagged, or operated, or that are heading to Israeli ports. However, many of the vessels that have been attacked have no connection with Israel. 

The Houthis originate from and operate out of Yemen, a country located in the southern end of the Arabian Peninsula. Yemen is bordered by Saudi Arabia to the north and Oman to the northeast. The capital of Yemen is Sanaa, one of the oldest cities in the world.

The Houthi rebels’ control significant territories within this Arabian nation, including Sanaa, and large portions of northern and western Yemen. They remain a major force in the decade-long Yemeni civil war. The conflict has led to a dire humanitarian crisis, with millions of people displaced and more in need of aid. 

What is the Islamic Resistance in Iraq?

The Islamic Resistance in Iraq is a loose coalition of Iranian-backed militias that oppose U.S. presence in Iraq and Syria. One group with that coalition, Kata’ib Hizballah, has claimed responsibility for attacks on U.S. military bases, including the recent deadly attack on a U.S. military base in Jordan. The group’s membership is deliberately vague, allowing each armed group a level of plausible deniability. There is evidence suggesting that Iran’s Revolutionary Guards play a coordinating role in the coalition. The group’s actions are part of efforts to drive U.S. troops out of the region, galvanized by the Israel-Hamas war. The U.S. has pledged to hold the responsible parties accountable, and there are concerns about the risk of military escalation in the region due to these attacks.

The Islamic Resistance in Iraq is part of the so-called “axis of resistance,” which also includes other Iran-linked groups in the region. These groups have carried out multiple attacks against U.S. forces in Iraq and Syria—and now Jordan—with the aim to push the U.S. to ramp up its Middle East defenses. The attacks have raised fears of regional escalation and have led to increased tensions between the involved parties.

What is Hamas?

Hamas, an acronym for Harakat al-Muqawama al-Islamiya (Islamic Resistance Movement), is a Palestinian Islamist political and military organization. It was established in 1987 and is dedicated to the destruction of Israel and the establishment of an independent Islamic state in the land of Israel.

Hamas is widely popular in Palestinian society due to its anti-Israeli stance and its promotion of Palestinian nationalism in an Islamic context. Since its founding, Hamas has been involved in ongoing attacks against Israeli civilians, including suicide bombings, indiscriminate rocket attacks, and other war crimes.

Hamas has a military wing known as the Izz al-Din al-Qassam Brigades. These militants are currently engaged in war with Israel and concentrated in the Gaza Strip and areas of the West Bank.

What is Hizballah?

Hizballah, also known as Hezbollah,  is a Lebanese Shia Islamist political and military organization that has state-like military capabilities including various missiles, rockets, and unmanned aircraft systems. The group is proficient in asymmetric and conventional tactics and has been involved in various attacks and operations throughout the Middle East. 

Hizballah was established in the early 1980s during the Lebanese Civil War. It arose with the financial and ideological support of Iran and aimed to establish an Islamic state in Lebanon based on Shia Islamic principles. The group’s ideology is heavily influenced by the Iranian revolution of 1979, which brought the Ayatollah Khomeini to power in Iran.

What actions has the Biden administration taken toward these groups?

The Biden administration has redesignated the Houthi rebels as a foreign terrorist organization. Kata’ib Hizballah has been on the list since 2009. Hamas and Hizballah since 1997.

The U.S. government maintains several lists related to terrorism, each serving distinct purposes and governed by different legal frameworks. These lists are tools for implementing and enforcing national security and foreign policy strategies. The most prominent of these are the Foreign Terrorist Organizations (FTO) and the Specially Designated Global Terrorists (SDGT). 

The FTO is a list managed by the State Department that designates foreign organizations as terrorist groups. Designation as an FTO makes it illegal for persons in the U.S. or subject to U.S. jurisdiction to knowingly provide “material support or resources” to the designated organization. It also allows for the freezing of the group’s assets in U.S. financial institutions and denies entry into the U.S. to representatives and members of these organizations.

In contrast, the SDGT is managed by the Treasury Department, specifically under the Office of Foreign Assets Control (OFAC). This list, which targets individuals and groups worldwide, focuses on cutting off financial support to terrorists. It includes a broader range of entities compared to the FTO list including individuals, groups, and companies involved in terrorist activities. Those designated are subjected to asset freezes and travel bans.

The Treasury Department recently sanctioned entities and individuals associated with Kata’ib Hizballah, highlighting the ongoing efforts to target the group’s financial network and its supporters.

President Biden removed the Trump administration’s designation of the Houthi rebels as terrorists, arguing it hampered humanitarian assistance to people in Yemen. A U.S. official told Axios that the administration believes the “Specially Designated Global Terrorist” designation is the “appropriate tool at this moment.”

For the past few weeks, the U.S. and other Western countries have also been carrying out military strikes against Houthi sites in Yemen in retaliation for the attacks on shipping.