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11:59 p.m. ET, December 31, 2023

What do we know about the Iran-backed Houthi rebel group?

From CNN's Christian Edwards: Iran-backed Houthi rebels in Yemen are stepping up attacks on ships in the Red Sea in what they say is revenge on Israel for its military action in the Gaza Strip. There are fears the attacks could escalate Israel's war against Hamas into a larger regional conflict. The U.S. said on Sunday that it had no desire to enter a major war but was acting in its own self-defense after U.S. Navy helicopters sank three Houthi boats that the U.S. said were aimed at aircraft in the Red Sea had shot. The helicopters had responded to a distress call from the latest merchant ship to come under Houthi fire. The attacks have forced some of the world's largest shipping and oil companies to halt transit through one of the world's most important maritime trade routes, potentially triggering a shock to the global economy.

Here's what we know about the Houthis:

Who are the Houthis? The Houthi movement, also known as Ansarallah (Supporters of God), is one side of Yemen's civil war that has been raging for nearly a decade. It emerged in the 1990s when its leader Hussein al-Houthi launched Believing Youth, a religious renewal movement for a centuries-old branch of Shiite Islam called Zaidism.

His closest followers became known as Houthis.

How did they gain power? Ali Abdullah Saleh, the first president of Yemen after the unification of North and South Yemen in 1990, initially supported the faithful youth. But as the movement's popularity grew and anti-government rhetoric increased, it became a threat to Saleh. Things came to a head in 2003 when Saleh supported the US invasion of Iraq, which many Yemenis opposed.

For al-Houthi, the break was an opportunity. He took advantage of public outrage and organized mass demonstrations. After months of unrest, Saleh issued an arrest warrant against him.

Al-Houthi was killed by Yemeni forces in September 2004, but his movement lived on. The Houthi military wing grew as more fighters joined the cause. Emboldened by early Arab Spring protests in 2011, they seized control of the northern province of Saada and demanded the end of the Saleh regime.

How powerful are the Houthis? American officials have pursued incremental improvements in the range, accuracy and lethality of the Houthis' domestically-made missiles. Initially, homemade Houthi weapons were assembled largely from Iranian components that were smuggled into Yemen in parts, an official familiar with US intelligence previously told CNN. But they have made modifications that have resulted in big improvements overall, the official said. In a novel development, the Houthis have used medium-range ballistic missiles against Israel, firing a volley of projectiles into Israel's southern Eilat region in early December. Israel said it intercepted the rockets.

While the Houthis may not pose a serious threat to Israel, their technology can wreak havoc in the Red Sea. They have used drones and anti-ship missiles to attack merchant ships – some believed to have no connection to Israel – prompting the US Navy and other warships in the Red Sea to respond to distress calls.

Read more about the Houthis.