The UK and US are launching air strikes at Houthi targets in Yemen in an effort to wipe out Iran-backed fighters attacking commercial ships in the Red Sea.
Here we answer some of the burning questions about who they are - and what exactly they want.
Who are the Houthis?
They are an armed political and religious group which champions Yemen’s Shia Muslim minority, the Zaidis. The group emerged in the 1990s and takes its name from late founder, Hussein al-Houthi.
What do they want?
Their aim is to govern all of Yemen, but they also declared themselves part of the Iran-led “axis of resistance” against Israel, the US and the West, along with groups such as Hamas and Lebanon’s Hezbollah.
Why are the Houthis attacking ships in the Red Sea?
They say they are targeting ships which are Israeli-owned, flagged or operated, or which are heading to Israeli ports, in solidarity with Palestine after Israel’s attacks in Gaza.
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Joe Biden said the strikes were in “direct response” to the attacks on Red Sea ships, which “jeopardised trade, and threatened freedom of navigation”. Rishi Sunak said it was “necessary” to protect global shipping.
What targets were hit and how damaging are the strikes?
Joint UK-US strikes on Saturday targeted 36 Houthi positions in 13 locations in Yemen, including underground storage facilities, command and control centres, missile systems, drone storage and operations sites, radars, and helicopters.
The US Central Command said it also hit a Houthi anti-ship missile in the Red Sea on Sunday. The Houthi military wing, known as Ansarallah, said it was targeted by 48 attacks, including 13 on the capital Sanaa.
Is there a danger of escalation?
This strategy pits the US against Iran, but experts stress that neither Washington nor Tehran want to slip into direct conflict. Iran, however, has set its red line by telling the US not to mount any direct attack on Iranian soil.
The Houthis have vowed to “meet escalation with escalation”, warning: “Either there is peace for us, Palestine and Gaza, or there is no peace and no safety for you in our region.”
How are other countries reacting to the strikes?
President Recep Tayyip Erdogan of Turkey said the US and UK were “trying to turn the Red Sea into a sea of blood”, Iran said the strikes were a “violation of Yemen’s sovereignty.. and a breach of international laws”.
Even Saudi Arabia has called for restraint. Countries in Europe have mostly been supportive, although Spain warned any country intervening militarily in the region “has to give explanations for its action”.
How could this pan out?
The crisis could drag on for months, and threaten a wider regional conflict if red lines are crossed.
Because of the low cost of drones and missiles, the attacks on ships are unlikely to stop, with disruption to commercial shipping through the Red Sea and significant economic cost for the countries which rely on it.