The United States and a handful of allies, including Britain, have struck back, carrying out missile strikes on Houthi targets inside Yemen and thrusting the militia and its long-running armed struggle further into the limelight. Last month, the State Department designated the Houthis as a terrorist organization, following through on warnings to crack down on the group.

Here’s a primer on the Houthis and their attacks on ships in the Red Sea.

Who are the Houthis?

The Houthis, led by Abdul-Malik al-Houthi, are an Iran-backed group of Shiite militants who have been fighting Yemen’s government for about two decades and now control the country’s northwest and its capital, Sana.

They have built their ideology around opposition to Israel and the United States, seeing themselves as part of the Iranian-led “axis of resistance,” along with Hamas in the Gaza Strip and Hezbollah in Lebanon. Their leaders often draw parallels between the American-made bombs used to pummel their forces in Yemen and the arms sent to Israel and used in Gaza.

In 2014, a military coalition led by Saudi Arabia intervened to try to restore the country’s original government after the Houthis seized the capital, starting a civil war that has killed hundreds of thousands.

Last April, talks between the Houthis and Saudi Arabia raised hopes for a peace deal that would potentially recognize the Houthis’ right to govern northern Yemen.

Once a group of poorly organized rebels, the Houthis have bolstered their arsenal in recent years, and it now includes cruise and ballistic missiles and long-range drones. Analysts credit this expansion to support from Iran, which has supplied militias across the Middle East to expand its own influence.

Why are they attacking ships in the Red Sea?

When the Israel-Hamas war started on Oct. 7, the Houthis declared their support for the people of Gaza and said they would target any ship traveling to Israel or leaving it.

Yahya Sarea, a Houthi spokesman, has said frequently that the group is attacking ships to protest the “killing, destruction and siege” in Gaza and to stand in solidarity with the Palestinian people.

The Gazan authorities say that more than 30,000 people, most of them civilians, have been killed in the Israeli bombing campaign and ground offensive that started after Hamas carried out cross-border raids and killed, the Israeli authorities say, about 1,200 people.

While the Houthis initially pledged to target all ships with links to Israel, they have since said their attacks are also in retaliation to the “American-British aggression” against them. Most ships that have been attacked have no obvious links to Israel and have not been bound for Israeli ports.

Since November, the Houthis have launched dozens of attacks with drones and missiles on vessels in the Red Sea and the Gulf of Aden.

The latest was on Wednesday, when the Houthis claimed an attack on a commercial vessel off the coast of Yemen that killed two people and injured at least six others, according to Western officials. The attack marked the first fatalities from Houthi attacks since the group began targeting ships.

How have the attacks affecting countries around the world?

Speaking to reporters in Bahrain on Jan. 10, the American secretary of state, Antony J. Blinken, warned that continued Houthi attacks in the Red Sea could disrupt supply chains and in turn increase costs for everyday goods. The Houthis’ attacks have affected ships tied to more than 40 countries, he said.

Shipping companies have been left with difficult options.

Rerouting vessels around Africa adds an extra 4,000 miles and 10 days to shipping routes, and requires more fuel. But continuing to use the Red Sea would raise insurance premiums. Either option would bruise an already fragile global economy.

In addition to holding critical shipping lanes, the waters off Yemen are a critical location for undersea cables that carry email and other digital traffic between Asia and the West. Three of these cables were disabled on Tuesday, raising concerns about whether the conflict in the Middle East is now beginning to threaten the global internet. The cause of the damage is still unclear, but suspicion has centered on the Houthis, who have denied responsibility.

What has the U.S. been doing to stop the Houthi attacks?

The Biden administration has repeatedly condemned Houthi attacks in the Red Sea and has assembled a naval task force to try to keep them in check.

The task force, called Operation Prosperity Guardian, brought together the United States, Britain and other allies and has been patrolling the Red Sea to, in Mr. Blinken’s words, “preserve freedom of navigation” and “freedom of shipping.”

Bahrain is the only Middle Eastern country that agreed to participate. Even though many countries in the region depend on trade that goes through the Red Sea, many do not want to be associated with the United States, Israel’s closest ally, analysts say.

U.S. and British warships have intercepted some Houthi missiles and drones before they reached their targets.

Last month, American and British warplanes hit 18 targets across eight locations in Yemen associated with Houthi underground weapons storage facilities, missile storage facilities, one-way attack unmanned aerial systems, air defense systems, radars and a helicopter.

The United States had earlier struck five Houthi military targets, including an undersea drone, in Houthi-controlled areas of Yemen.

In January, American fighter jets from the aircraft carrier U.S.S. Dwight D. Eisenhower, along with four other warships, intercepted 18 drones, two anti-ship cruise missiles and one anti-ship ballistic missile, Central Command said in a statement. In December, U.S. Navy helicopters sank three Houthi boats that were attacking a commercial freighter.

Ben Hubbard, Peter Eavis, Helene Cooper, Eric Schmitt and Keith Bradsher contributed reporting.