The United States and Britain launched strikes from the air and sea against Houthi military targets in Yemen in response to the movement's attacks on ships in the Red Sea, a dramatic regional widening of the Israel-Hamas war in Gaza.
As witnesses in Yemen confirmed explosions throughout the country to Reuters, President Joe Biden cautioned in a statement late on Thursday he would not hesitate to take further action if needed.
Here are some details about the Iran-aligned group.
HISTORY
In the late 1990s, the Houthi family in far north 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 marginalised.
As friction with the government grew, they fought a series of guerrilla wars with the national army and a brief border conflict with Sunni Saudi Arabia.
WAR IN YEMEN
The war began in late 2014 when Sanaa was seized by the Houthis. Worried by the growing influence of Shi'ite Iran along its border, Saudi Arabia intervened at the head of a Western-backed coalition in March 2015 in support of the Saudi-backed government.
The Houthis established control over much of the north and other big population centres, while the internationally recognised government based itself in Aden.
Yemen has enjoyed more than a year of relative calm amid a U.N.-led peace push. Saudi Arabia has been holding talks with the Houthis in a bid to exit the war.
But the Houthi attacks on Israel have increased the risks of conflict for Saudi Arabia.
HOUTHI ATTACKS ON ISRAEL
The Houthis say their attacks on shipping routes in the Red Sea are a show of support for the Palestinians and Hamas, the Islamist group that controls Gaza, in its war against Israel.
The Houthi attacks have disrupted international commerce, forcing international shipping to take the long route around South Africa to avoid being struck. The increase in delivery costs is stoking fears it could trigger a fresh bout of global inflation.
The U.S. said Australia, Bahrain, Canada and the Netherlands supported the operation against the Houthis, and sought to present the air strikes as part of an international effort to restore the free flow of trade in a key route between Europe and Asia that accounts for about 15% of the world's shipping traffic.
WHAT IS THE AIM OF THE HOUTHI ATTACKS?
The Houthis are one part of what has been called the "Axis of Resistance" - an anti-Israel and anti-Western alliance of regional militias (Hamas, Hezbollah and Houthis) backed by Iran.
The Houthis' slogan is "Death to America, Death to Israel, curse the Jews and victory to Islam".
LINKS WITH IRAN
The Houthis have built ties with Iran, but it is not clear how deep that relationship goes. The Saudi-led coalition accuses Iran 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 as part of its regional "axis of resistance", Yemen experts say they are motivated primarily by a domestic agenda though they share a political affinity for Iran and Hezbollah. The Houthis deny being puppets of Iran and say they are fighting a corrupt system.
