Despite months of U.S.-led airstrikes against Yemen’s Houthi fighters. The once ragtag rebels have continued to threaten some of the world’s most vital shipping routes. Drawing from an arsenal of increasingly advanced weapons to attack vessels in and around the Red Sea.

Just this month, Houthi militants sank one ship and set another ablaze. The fighters, operating on land and in the water, have launched swarms of drones at U.S. warships. They also deployed a remote-controlled boat packed with explosives, tactics, and weapons. Experts say these weapons are associated with the group’s patron, Iran.

The recent uptick in Houthi activity has underscored the group’s ability to pose a sustained threat. The group relies in part on a steady flow of Iranian arms. They have the expertise to withstand U.S. strikes and remain on the attack. The faltering U.S. efforts to halt Houthi operations and protect global shipping have also drawn scrutiny from Congress. It is a place where lawmakers say not enough is being done to establish deterrence.

Iran’s Shadow War Fuels Houthi Attacks on Red Sea Shipping

For years, Iran has circumvented a United Nations arms embargo against Yemen. Covertly sending weapons and equipment from Iranian ports to the Arabian Sea or overland from neighboring Oman. The Houthis have also learned how to modify old weapons and manufacture new ones. According to senior U.S. military commanders, they became the first group to use anti-ship ballistic missiles to strike naval targets.

The Houthi movement leaders represent a minority Shiite sect in northern Yemen. They first emerged in the 1990s and later seized the capital, Sanaa. They fought a bruising war with Saudi Arabia, which wanted to eliminate an Iranian proxy on its border, but ultimately stayed in power and expanded the amount of territory they controlled.

In Sanaa, Houthi officials took part in a protest of the United States and Israel and stood in solidarity with the Palestinian people earlier this month. (Yahya Arhab/EPA-EFE/Shutterstock)

Experts estimate that the group has a fighting force of at least 20,000, including a mix of tribal forces and troops formerly loyal to the government.

In November, after war broke out between Israel and Hamas, the Houthis announced they would begin attacking Israeli-linked ships in solidarity with Palestinians in Gaza. Their first major salvo included hijacking a cargo vessel in the southern Red Sea and detaining its crew.

Since then, the Pentagon has recorded more than 190 attacks on either U.S. military vessels or commercial shipping off the coast of Yemen, including nearly 100 since waves of U.S. airstrikes began in January.

The Houthis have sunk two ships, including the Rubymar in March and the Greek-owned Tutor coal carrier. It was hit in the stern last week by an explosives-filled surface vessel. Also in March, an anti-ship ballistic missile fired by the Houthis set the Barbados-flagged True Confidence on fire, killing three people.

Houthis Threaten Global Trade Route

The operations soon broadened to the Gulf of Aden and Bab el-Mandeb Strait, which connects the Indian Ocean to the Red Sea. From there, ships transit through the Suez Canal to the Mediterranean, the shortest maritime route between Europe and Asia.

The Houthis “will continue to understand that there’s a price to be paid” for harming regional maritime trade, according to Pentagon spokesman Maj. Gen. Patrick Ryder. He told reporters on Tuesday, calling the attacks “unacceptable.”

The Pentagon has deployed a rotating cast of warships in the region to thwart the Houthi threat. They have been shooting down drones over the Red Sea and other waterways and striking missiles and radar sites in Yemen.

The effort has included an aircraft carrier, the USS Dwight D. Eisenhower, and the destroyers and other warships deployed with it. The Eisenhower deployed in October and has seen its mission extended twice by Defense Secretary Lloyd Austin. As the Pentagon prioritizes keeping firepower in the Middle East.

On Saturday, U.S. Central Command said the Eisenhower carrier strike group was returning home after it “protected freedom of navigation throughout the Red Sea and Gulf of Aden.” The statement said the USS Theodore was deploying to the region after completing a scheduled exercise in the Indo-Pacific.

Fighter jets maneuver on the deck of the USS Dwight D. Eisenhower in the Red Sea on June 11. (Bernat Armangue/A.P.)

However, Republican lawmakers, some pushing for a dramatic surge in Pentagon spending in the year ahead. They have accused the Biden administration of underinvesting in the advanced weapons and surveillance technology they now say are necessary for the fight.

U.S. Navy Battles Houthi Attacks, Iran Blamed

“We just simply don’t have the political will to go after them,” Sen. Mike Rounds (R-S.D.). He sits on the Senate Intelligence and Armed Services committees, said in an interview Tuesday.

He attributed the rise in Houthi attacks to “resources that are being directed to them by Iran,” as well as “enhanced technology that has made their systems more accurate.”

“Each of the different system types has their own capability,” said Rounds, who declined to comment on specific weaponry. “I don’t want to get into what it is that’s the most significant, but it’s more advanced than what they had to begin with,” he said.

The administration in March said that it was expanding efforts to intercept Iranian weapons being smuggled to Yemen. On Monday, the Treasury Department’s Office of Foreign Assets Control said it targeted several individuals and entities involved in weapons procurement for the Houthis with sanctions.

Sen. Mark Kelly (D-Ariz.) said that the U.S. naval destroyers and aircraft carrier battle groups in the region have been “rather successful” at disrupting attacks. U.S. forces, he said, have “expended a lot of munitions in order to protect shipping.”

“If we don’t protect that shipping. We are going to see increased supply chain problems,” Kelly, a Navy veteran who serves on the Senate Armed Services Committee, said in an interview Tuesday.

He said he had just reviewed classified intelligence on the issue and could not comment in detail on efforts to disrupt Iranian weapons shipments to the Houthis. But he acknowledged that the Houthis were continuing to acquire advanced weaponry from Iran.

“I think as they get munitions from the Iranians that they feel it’s in their best interest to use them in disruption in the Red Sea,” Kelly said.

Houthis Tighten Grip in Yemen

The Houthis’ relative success in their Red Sea campaign has given them the flexibility to maneuver more easily in the region and at home.

“This is an attempt to demonstrate that the Houthis are a serious regional actor,” said Hannah Porter. A Yemen researcher with ARK Group, a U.K.-based international development organization. After engaging in direct combat with the U.S. military, Porter said the Houthis “can now portray themselves as a power player”. They can use that to tighten their grip domestically or in ongoing peace talks with Saudi Arabia.

That work is already underway on the ground in Yemen. Images from the conflict, including video of the November hijacking and missile strikes on other vessels, are used by the Houthis. According to researchers and local media reports, it is important to both drive recruiting campaigns and crack down on dissent. Houthi media outlets have reported that tens of thousands of additional fighters have joined their ranks since the Red Sea attacks began.

“The Houthis are very good at seizing opportunities to assert themselves,” said Nadwa al-Dawsari. A Yemeni researcher based in the United States with the Middle East Institute. “And in this case, they are using the Red Sea attacks to gear up for escalation in Yemen.”

Earlier this month, the Houthis launched a widening crackdown on abducting aid workers with the United Nations and the Washington-based National Democratic Institute.

Dawsari said the arrests are aimed at extinguishing the small pockets of dissent that remain in Houthi-controlled Yemen. “These voices have been suppressed, but now the Houthis want to eliminate them completely,” she said.