Explainer-Who are Yemen's Houthis and why are they under attack by the US?

U.S. President Donald Trump has launched the biggest military operation in the Middle East since taking office against Yemen's Iran-aligned Houthis after they resumed Red Sea shipping attacks.

The strikes, which have killed at least 24 people so far, could last for weeks according to one U.S. official and come as Trump ramps up sanctions pressure on Tehran.

WHAT IS THE HISTORY OF THE HOUTHIS?

In the late 1990s, the Houthi family in the far north of Yemen set up a religious revival movement for the Zaydi sect of Shi'ite Islam, which had once ruled Yemen, but whose northern heartland had became impoverished and marginalized.

As friction with the government in the capital Sanaa grew, they fought a series of guerrilla wars with the national army and a brief border conflict with Sunni Muslim Saudi Arabia.

WHO IS THE HOUTHI LEADER?

Abdul Malik al-Houthi created the defiant force challenging world powers from a group of ragtag mountain fighters in sandals. Al-Houthi established a reputation as a fierce battlefield commander before emerging as head of the Houthi movement.

Under the direction of al-Houthi, who is in his 40s, the group has grown into an army of tens of thousands of fighters and acquired a huge arsenal of armed drones and ballistic missiles. Saudi Arabia and the West say the arms come from Iran, though Tehran denies this.

Al-Houthi is known for rarely staying long in one place, for never meeting the media and for extreme reluctance to make scheduled public appearances.

HOW DID THE HOUTHIS COME TO CONTROL MUCH OF YEMEN?

A civil war erupted in Yemen in late 2014 when the Houthis seized Sanaa. Worried by the growing influence of Shi'ite Iran along its border, Saudi Arabia led a Western-backed coalition in March 2015, which intervened in support of the Saudi-backed government.

The Houthis established control over much of the north and other large population centres, while the internationally recognised government based itself in the port city of Aden.

Yemen enjoyed a period of relative calm amid a U.N.-led peace push, but a sharp escalation in regional tensions since the start of the Gaza war has increased risks of a new conflict between the militia and Riyadh.

WHY ATTACK SHIPS?

The Houthis have waded into the Gaza conflict with attacks on shipping routes in the Red Sea as a show of support for the Palestinians and Hamas, the Islamist group that controls Gaza.

They have also claimed attacks on ships they say are linked to Israel in the Gulf of Aden, the Arabian Sea, the Indian Ocean and the Mediterranean Sea.

The Houthi attacks have disrupted global shipping, forcing firms to re-route to longer and more expensive journeys around southern Africa.

While the militia has said it attacked only vessels with links to Israel, the United States and Britain, shipping industry sources say all ships were at risk.

The U.S. and Britain had retaliated with air strikes against the Houthis as part of international efforts to restore the free flow of trade along a key route between Europe and Asia that accounts for about 15% of the world's shipping traffic.

A period of relative calm started in January alongside the Gaza ceasefire but the group then warned in March it would resume its naval operations if Israel did not lift a blockage of aid into Gaza.

The Houthis then announced on March 12 a resumption of attacks with immediate effect.

WHAT ARE THE HOUTHIS' LINKS WITH IRAN?

The Houthis are part of what has been called the "Axis of Resistance" - an anti-Israel and anti-Western alliance of regional militias including Hamas, Hezbollah and the Houthis, backed by Iran.

The Houthis' slogan is "Death to America, Death to Israel, curse the Jews and victory to Islam".

The Saudi-led coalition accuses Tehran of arming and training the Houthis, a charge both deny. The coalition also says Lebanon's Iran-backed Hezbollah is helping the Houthis, an accusation it rejects.

While Iran champions the Houthis, the Houthis deny being puppets of Iran, and Yemen experts say they are motivated primarily by a domestic agenda.

(Editing by Peter Graff and Lincoln Feast.)