Houthis Expand Underground Bases—That Could Be a Big Problem for the US

The Iran-aligned rebel movement in control of much of Yemen has expanded underground military infrastructure in a bid to boost resilience in the face of clashes with U.S. forces and the prospect of an all-out regional conflict erupting over Israel's conflict in Gaza.

While Ansar Allah, also known as the Houthi movement, has long been known to utilize subterranean bases throughout Yemen's decade-long civil war, the expansion of this network amid worsening Middle East tensions can be seen in recent satellite imagery. Tunnels to be used for waging war have also been featured in recent videos released by the group and one senior militia official has stated that there is much more in store than what has been uncovered thus far.

"Regarding the tunnels, it is nothing new to our military structure, and what was presented is negligible in the face of the infrastructure that we possess because they are just small tunnels for individuals," Ansar Allah Deputy Information Secretary Nasreddin Amer told Newsweek.

"However," he added, "what was not presented is an integrated military structure that is considered one of the most important factors of our military steadfastness."

Fabian Hinz, a senior research fellow at the London-based International Institute for Strategic Studies (IISS) who recently analyzed new satellite imagery of underground Ansar Allah bases in Yemen, also noted the likely value of such infrastructure to the group's strategy and capabilities.

"It's unclear how extensive they are," Hinz told Newsweek. "But it's fair enough to say that they're making a substantial investment on underground facilities, and that is probably a key factor in their warfighting doctrine."

Tunnels Take Root in Yemen

What Hinz has revealed through satellite images are signs of new construction and tunnel entrances at underground facilities in the heart of Saada province that borders Saudi Arabia, at the Al-Hafa base on the eastern outskirts of Sanaa and at the Jabal Attan missile base just west of the capital city.

As Hinz noted in his report, the origins of underground facilities in Yemen date back to the rule of President Ali Abdullah Saleh, who led for nearly 25 years, first ruling North Yemen and then the unified nation until his ousting amid widespread protests in 2012. Tensions between the government and Ansar Allah worsened in Saleh's final decade in power, and the group utilized natural cave systems in the north to evade troops.

It was reportedly in one such cave in Saada that Yemeni security forces killed the group's leader, Zaidi Shiite Muslim cleric Hussein al-Houthi, in 2004. His brother, Abdul-Malik al-Houthi, would then lead the group to advance on the capital a decade later amid nationwide instability that followed Saleh's downfall during the regional Arab Spring protest movement.

Ansar Allah began to invest further in underground capabilities as the group sought to resist a Saudi-led coalition campaign of airstrikes aimed at restoring the country's internationally recognized government. Among the targets of this aerial offensive were underground facilities, but Ansar Allah pressed on with reconstruction and refurbishment, challenging conventional notions of anti-tunnel combat.

"For a long time, people have been saying in the era of precision air warfare you can't really use on the ground facilities," Hinz said, "because they will just bomb the entrances and the air vents and then they will turn into a grave for whoever's inside and you can destroy them."

"The Houthis apparently see this quite differently," he added. "They've managed to refurbish underground facilities that have been destroyed or with entrances that have been destroyed by the Saudi-led coalition. So, they still see this as a viable path forward."

Shifting Frontlines in the Middle East

Fighting among Yemen's warring factions has subsided significantly since a United Nations-brokered ceasefire reached two years ago and a subsequent China-mediated deal to reestablish relations between Iran and Saudi Arabia in March of last year. But Ansar Allah now faces an aerial threat from a new foe, the United States.

The eruption of the deadliest-ever conflict in Gaza nearly seven months ago served as a catalyst for a number of Iran-aligned "Axis of Resistance" factions to launch campaigns across the region, including from Lebanon, Iraq, Syria and Yemen. In the case of Ansar Allah, the group has fired missiles and drones a thousand miles away toward Israel and has mounted an unprecedented maritime offensive against commercial ships, as well as U.S. forces, in the Red Sea and surrounding waters.

The U.S. has responded with repeated rounds of airstrikes in partnership with the United Kingdom against Ansar Allah positions in Yemen, including underground sites. And yet, the group has continued to launch near-daily attacks on Israel and international vessels, vowing not to cease unless Israel halted its campaign against the Palestinian Hamas movement, whose October 7, 2023, attack sparked the war in Gaza.

Newsweek has reached out to U.S. Central Command for comment.

Hamas, too, has made use of intricate tunnels and compounds underneath the streets of densely populated Gaza to supplement its combat against Israel. Across the border in Lebanon, Hezbollah has long established a robust subsurface network as well.

Iran, for its part, has gone so far as to establish vast underground complexes that shield its nuclear program, which the country repeatedly denies is geared toward developing a nuclear weapon, and to host sprawling so-called "missile cities." A number of these missiles and drones were used to strike Israel directly last month in response to Israel's earlier targeting of a consular building attached to the Islamic Republic's embassy in Syria.

Ansar Allah joined in on the historic retaliatory salvo on Israel, and the group's own underground facilities may be used to support future attacks.

Hinz said that the underground facilities he has observed under Ansar Allah's control include entrances large enough to accommodate vehicles, suggesting they could potentially host strategic assets such as drones, ballistic missiles, cruise missiles, and anti-ship missiles. He argued that these sites may also support the production of such weapons.

Fears of the Unknown

Walker Mills, a U.S. Marine Corps officer who has studied and offered his analysis on tunnel warfare, also observed that Ansar Allah's subterranean structures would likely be used to store and build weapons and protect key elements of the group's military infrastructure, including supply depots and control nodes.

They may also be used to enhance offensive operations, as well.

"The Houthis could prepare their weapons for firing inside these facilities, pop out and shoot, and then go back inside," Mills told Newsweek. "That would limit their vulnerability to strikes and also limit the warning of strikes. But being able to do that depends on how the facilities are being constructed and where they are."

When it comes to underground infrastructure, he noted that what isn't known presents the greatest challenge.

"It can be difficult to hide the presence of underground facilities, but what exactly their size, shape and character is, let alone what is happening inside of them, is very difficult, if not impossible, to determine," Mills said. "Going underground is a tried-and-true way to keep your actions and equipment away from prying eyes, and it has been successful from Vietnam and North Korea to Afghanistan."

The U.S. has experience tackling foes underground in all three countries—to varying degrees of success. In recent years, the Pentagon has also launched several subterranean combat training initiatives and invested in more robust bunker-busting capabilities, such as the 30,000-pound GBU-57A/B Massive Ordnance Penetrator (MOP) that can be fitted to the B-2 Spirit stealth bomber and other aerial assets.

However, such operations present unique logistical hurdles that would demand substantial resources, and the opaque nature of the facilities in question could limit the ability to assess the success of attacks.

"Basically, if you want to destroy underground facilities you probably need to use different bombs and different aircraft—that complicates your strikes," Mills said. "And then, going back to the first point, it also makes it very difficult to know if your strikes were effective or not once you strike a target."

A Looming Showdown

Just how far the White House is willing to take the fight against Ansar Allah remains to be seen. President Joe Biden has repeatedly stated that he did not seek to escalate the crises already gripping the Middle East on the sidelines of the war in Gaza, and Yemen has rarely been cited as a priority for potential ground operations.

How long U.S. forces will sustain limited yet costly operations without a break in Ansar Allah attacks also remains a matter of debate. The group hailed the shootdown of yet another MQ-9 Reaper attack drone over Yemen on Friday, marking the third such loss for the U.S. military since the war in Gaza began.

Both factors threaten to be further influenced by the increasingly direct confrontation between Iran and Israel, which reportedly conducted a limited missile strike on Iran days after the Islamic Republic's barrage. As Amer recently told Newsweek, Ansar Allah has committed to aiding Tehran and other Axis of Resistance allies in the event of a full-scale regional conflict.

"As for our defense of ourselves, our region, and those with whom we have alliances, such as the Axis of Resistance," Amer said, "we will certainly never stand idly by in the face of any foreign, Western aggression against any country in our Islamic world."

"Even if it is one of those countries with which we disagree, we will never leave it alone," he added. "So, when it comes to those with whom we have alliances, such as the Islamic Republic of Iran and other parties of the Axis of Jihad and Resistance, we will certainly stand with all strength and determination alongside each other, and these are basically the teachings of the Quran."