Who are the Houthis and why hasn't the US retaliated for their attacks on ships?

When Iranian-backed Houthi rebels in Yemen launched missiles and hit three commercial ships in the southern Red Sea last weekend, it triggered an immediate question: Will the U.S. military strike back?

The Houthis have sharply escalated their attacks against ships as they sail toward the narrow Bab el-Mandeb Strait. And U.S. Navy ships have shot down an array of drones headed their way and believed to have been launched by the militant group from territory it controls in Yemen.

But so far, the U.S. has avoided military retaliation — a marked difference from its multiple strikes against Iran-backed militias in Iraq and Syria that have fired rockets, missiles and drones at bases housing American forces in both countries.

No one has been reported hurt in the Houthi incidents, although the commercial ships suffered some damage. And U.S. officials argue that the Houthis haven't technically targeted U.S. vessels or forces — a subtlety that Navy ship captains watching the incoming drones may question.

Here's a look at the Houthis and their increasing attacks, and why the U.S. believes it is more acceptable to bomb some Iranian-linked targets than others.

WHO ARE THE HOUTHIS AND WHAT'S GOING ON IN YEMEN

Houthi rebels swept down from their northern stronghold in Yemen and seized the capital, Sanaa, in 2014, launching a grinding war. A Saudi-led coalition intervened in 2015 to try to restore Yemen's exiled, internationally recognized government to power.

Years of bloody, inconclusive fighting against the Saudi-led coalition settled into a stalemated proxy war between Saudi Arabia and Iran, causing widespread hunger and misery in Yemen, the Arab world's poorest country. The war has killed more than 150,000 people, including fighters and civilians, and created one of the world's worst humanitarian disasters, killing tens of thousands more.

A cease-fire that technically ended more than a year ago is still largely being honored. Saudi Arabia and the rebels have done some prisoner swaps, and a Houthi delegation was invited to high-level peace talks in Riyadh in September as part of a wider détente the kingdom has reached with Iran. While they reported "positive results," there is still no permanent peace.

ATTACKS ON SHIPS

The Houthis have sporadically targeted ships in the region over time, but the attacks have increased since the start of the war between Israel and Hamas and spiked after an explosion Oct. 17 at a hospital in Gaza killed and injured many. Houthi leaders have insisted Israel is their target.

After the weekend attacks, Houthi military spokesman Brig. Gen. Yahya Saree said the group wants to "prevent Israeli ships from navigating the Red Sea (and Gulf of Aden) until the Israeli aggression against our steadfast brothers in the Gaza Strip stops."

One of the commercial ships hit on Sunday — the Unity Explorer — has a tenuous Israeli link. It is owned by a British firm that includes Dan David Ungar, who lives in Israel, as one of its officers. Israeli media identified Ungar as being the son of Israeli shipping billionaire Abraham "Rami" Ungar. But any Israel connections to other ships are unclear.

Sunday's flurry of attacks included missiles that hit the Unity Explorer, the Number 9 and the Sophie II, all bulk carriers. And throughout that day, the USS Carney, a Navy destroyer, shot down three drones that were headed toward the ship and also went to the aid of the commercial vessels. On Wednesday, the USS Mason shot down a drone heading in its direction.

In a statement, U.S. Central Command said, "We cannot assess at this time whether the Carney was a target" of the drones.

THE US CALCULUS

While the U.S. has carried out airstrikes on Iranian-back militias in Iraq and Syria that have targeted American troops in 77 different attacks since Oct. 17, the military has not yet retaliated against the Houthis.

That reluctance reflects political sensitivities and stems largely from broader Biden administration concerns about upending the shaky truce in Yemen and triggering a wider conflict in the region. The White House wants to preserve the truce and is wary of taking action that could open up another war front.

U.S. officials warn that military action is an option and they haven't taken it off the table. But both publicly and privately, officials stress that there is a difference between the Iraq and Syria bombings and the Houthi attacks.

Iran-backed militia have launched one-way attack drones, rockets or close-range ballistic missiles at bases in Iraq 37 times and in Syria 40 times. Dozens of troops have suffered minor injuries — in most cases traumatic brain injuries. In all instances so far, the personnel have returned to work.

In response, the U.S. has retaliated with airstrikes three times in Syria since Oct. 17, targeting weapons depots and other facilities linked directly to Iran's Revolutionary Guard Corps and the militias. And it struck multiple sites in Iraq late last month after a militia group for the first time fired short-range ballistic missiles at U.S. forces at al Asad air base.

The Houthis, meanwhile, have fired missiles at vessels in the Red Sea, launched drones and missiles targeting Israel and sent drones in the direction of Navy ships. Also, last month, Houthis seized a vehicle transport ship linked to Israel in the Red Sea off Yemen, and still hold the vessel. And Houthi missiles landed near another U.S. warship after it assisted a vessel linked to Israel that had briefly been seized by gunmen.

Defending the lack of retaliation for those attacks has forced U.S. officials to dance on the head of a pin.

In one breath, the Pentagon officials say the Navy ships shot down the Houthi drones heading toward them because they were deemed "a threat." But in the next breath officials say the U.S. assesses that the ships were not the target. That determination often comes later after intelligence assessments review telemetry and other data.

That, however, is certainly no comfort to sailors on the ships who watch the radar track of incoming drones and must make rapid decisions about whether it represents a threat to the ship.

At the same time, the U.S. has consistently said it wants to protect free navigation of the seas. But the Houthi actions have prompted the International Maritime Security Construct to issue a warning for ships transiting the Red Sea and Bab el-Mandeb. It says ships should choose routes as far from Yemeni waters as possible, travel at night and not stop, because that makes them an easier target.

This week the U.S. said it was talking with allies about using a naval task force to escort commercial ships in the Red Sea. About 38 countries participate in a similar task force in the region — largely to battle piracy off the coast of Somalia. Officials have to discuss the issue with allies to see who wants to be involved in a new effort.

ESCALATION?

The Biden administration has talked persistently about the need to avoid escalating the Israel-Hamas war into a broader regional conflict. So far, strikes on the Iranian-backed groups in Iraq and Syria have not broadened the conflict, said Maj. Gen. Pat Ryder, Pentagon press secretary.

So it's not clear if targeted strikes against Houthi weapons depots or similar sites — which also have Iranian support — would cross a line and trigger a wider war.

"We will continue to consult with international allies and partners on an appropriate way to protect commercial shipping going through that region, and at the same time ensuring we do what we need to do to protect our forces," said Ryder.