The US has launched its biggest military operation in the Middle East against Yemen's Iran-aligned Houthis since Donald Trump took office.
The strike was launched over the group's attacks against Red Sea shipping, killing at least 53 people at the start of a campaign that could last for weeks according to a US official.
Five children and two women were among those killed, while 98 others were injured, according to the Houthi-run health ministry.
Mr Trump warned Iran, the Houthis' main backer, that it needed to immediately halt support for the group.
He said if Iran threatened the United States, "America will hold you fully accountable and, we won't be nice about it!"
Houthi leader Abdul Malik al-Houthi said on Sunday that his militants would target US ships in the Red Sea as long as the US continued its attacks on Yemen.
"If they continue their aggression, we will continue the escalation," he said in a televised speech.
So, how did we get here?
Firstly, who are the Houthis?
The Houthis are a Shiite Muslim rebel group that overran Yemen's capital, Sana'a, in 2014 and have controlled much of the country throughout the ongoing civil war.
As Reuters has previously reported, in the late 1990s the Houthi family in far north Yemen "set up a religious revival movement for the Zaydi sect of Shiite Islam, which had once ruled Yemen but whose northern heartland had become impoverished and marginalised".
As friction with the government in Sana'a grew, they fought a series of guerilla wars with the national army and a brief border conflict with Sunni Muslim Saudi Arabia.
Dr Sarah Phillips, a professor of global conflict and development at the University of Sydney, said the group's only claim to legitimacy by the people they ruled was their ability to claim they were defending Palestinians and fighting against America.
"Being able to reveal themselves as the ones who are willing to take the fight to Israel has been one to really bolster their legitimacy throughout the region, and also within Yemen," she told the ABC.
"People are really struggling in Yemen, and their continued emphasis on what they are doing outside of Yemen's borders is an attempt of a distraction for the local audience."
A history of the US and the Houthis
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.
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 US 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 per cent 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.
In response, Mr Trump said he ordered a series of air strikes on the Houthi-held areas in Yemen, promising to use "overwhelming lethal force" until the rebels ceased their attacks on shipping along the vital maritime corridor.
"Our brave Warfighters are right now carrying out aerial attacks on the terrorists' bases, leaders, and missile defenses to protect American shipping, air, and naval assets, and to restore Navigational Freedom," Mr Trump said in a social media post.
"No terrorist force will stop American commercial and naval vessels from freely sailing the Waterways of the World."
He also warned Iran to stop supporting the rebel group, promising to hold the country "fully accountable" for the actions of its proxy.
Houthis 'unlikely' to respond in the way Trump wants, expert says
In response to Trump's attacks, the top commander of Iran's Revolutionary Guards, Hossein Salami, said the Houthis were independent and took their own strategic and operational decisions.
"We warn our enemies that Iran will respond decisively and destructively if they take their threats into action," Major General Salami told state media.
Dr Sarah Phillip said the attack from Mr Trump was an attempt to "squeeze" the Houthis, but suspected they were "unlikely" to respond in the way in which Mr Trump would be hoping and she did not expect the group's behaviour to change.
"This is an organisation that have been fighting since 2004 … they are very accustomed to taking hits like this," she said.
"They are very good at moving their equipment around, they are good at sustaining causalities, and they are also fairly unconcerned about sustaining civilian causalities."
She said they were going to be prepared to respond and adapt for a long time.
"The question is more what the US and what Saudi Arabia decides to do," she said.
United Nations Secretary-General António Guterres on Sunday called for "utmost restraint and a cessation of all military activities" in Yemen.
He also warned new escalation could "fuel cycles of retaliation that may further destabilise Yemen and the region, and pose grave risks to the already dire humanitarian situation in the country," his spokesperson said in a statement.
US Secretary of State Marco Rubio told CBS News there was "no way the Houthis would have the ability to do this kind of thing unless they had support from Iran".
"And so this was a message to Iran: don't keep supporting them, because then you will also be responsible for what they are doing in attacking navy ships and attacking global shipping," he said.
ABC/Wires
