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.
The Iran-aligned Houthis are in a campaign they say aims to support Palestinians in the Gaza war.
The Houthis' role has added to the conflict's regional risks, , and worrying states on the Red Sea as Houthi rockets and drones fly towards Israel.
Who are the Houthis?
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 become 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 powerhouse Saudi Arabia.
When did the war in Yemen start?
Their power grew during the Yemen war which began in late 2014, when they seized Sanaa. Worried by the growing influence of Shi'ite Iran along its border, Saudi Arabia intervened at the head of a Western-backed coalition in 2015 in support of the Yemeni 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 UN-led peace push. Saudi Arabia has been holding talks with the Houthis in a bid to exit the war.
The Houthis media centre supplied photos of their manoeuver on Israel. Source: AAP / EPA
When and why did the Houthis attack Israel?
The Houthis waded into the latest conflict as it spread around the Middle East, announcing on 31 October they had fired drones and missiles at Israel and vowing they would continue to mount attacks "until the Israeli aggression stops".
Their actions have echoed the role of the , which has been attacking Israeli positions at the Lebanese frontier, and Iraqi militias which have been firing at US interests in Iraq and Syria.
Stepping up their threats, the Houthis said on 9 December they would target all ships heading to Israel, regardless of nationality, and warned all international shipping companies against dealing with Israeli ports.
"If Gaza does not receive the food and medicine it needs, all ships in the Red Sea bound for Israeli ports, regardless of their nationality, will become a target for our armed forces," a Houthi spokesperson said in a statement.
The Houthis' slogan is "Death to America, Death to Israel, curse the Jews and victory to Islam".
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 US 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 per cent of the world's shipping traffic.
Are the Houthis linked to Iran?
The United States believes that Iran's Islamic Revolutionary Guard Corps is helping to plan and carry out the Houthi missile and drone attacks.
"Iran's support for Houthi attacks on commercial vessels must stop," Austin said on 18 December.
The Saudi-led coalition has long accused Iran of arming, training and funding the Houthis. The Houthis deny being an Iranian proxy and say they develop their own weapons.
What weapons do the Houthis have?
The Houthis demonstrated their missile and drone capabilities during the Yemen war in attacks on Saudi Arabia and the United Arab Emirates, targeting oil installations and vital infrastructure.
The arsenal includes ballistic missiles and armed drones capable of hitting Israel more than 2,000 kilometres from their seat of power in Sanaa.
Its Tofan, Borkan, and Quds missiles are modelled on Iranian weapons and can hit targets up to 2,000 km away, experts say.
The Houthis fired these missiles at Saudi Arabia dozens of times during the Yemen war. In September, the Houthis displayed anti-aircraft Barq-2 missiles, naval missiles, a Mig-29 fighter jet and helicopters for the first time.
The Houthis have also used fast boats armed with machine guns in their operations against shipping.
