For days, drones surveilled a desert base in the far reaches of Yemen where Saudi Arabian officers prepared to train newly enlisted local forces as the fight against the Iran-backed Houthi militia rages.

Just as the training got under way Oct. 19, the Houthis hit the base with three ballistic missiles over the span of a few minutes, killing at least 75 people and wounding more than 200 others—including Saudi troops—in the deadliest single attack by the militants in the new war, Saudi and Yemen officials briefed on the incident and witnesses said.

The strike on Shabwah was one of a string of recent attacks that show the stiff challenge Saudi Arabia faces in trying to contain an enemy that has demonstrated an ability to gather sensitive intelligence on its movements and respond quickly with precision.

On Oct. 15, a Houthi drone and missile attack targeted Al-Maashiq Palace in Aden, the seat of power for Yemen’s internationally recognized government, killing and injuring several people including Saudi officers, whose housing was hit by one of the missiles, some of the Saudi and Yemeni officials briefed on the incident said.

On Wednesday, a Houthi missile attack targeted an airport near the eastern port city of Mukalla shortly after Saudi military reinforcements reached a base there, some officials said.

“The bigger picture is that the Houthis are systematically targeting military reinforcements and redeployments across Yemen, including newly arriving vehicles, ammunition shipments, troop concentrations and gatherings at recruitment centers,” said Mohammed al-Basha, a Yemen expert who runs the U.S.-based Basha Report risk-advisory company. The government of Saudi Arabia didn’t respond to requests for comment. Saudi Arabia has tried and failed to contain the Houthi threat on its southern border for more than a decade.

The kingdom intervened in Yemen’s civil war in 2015 shortly after the Houthis seized the capital, San’a, but struck a ceasefire seven years later after the Houthis hit Saudi cities and energy infrastructure.

The kingdom offered financial support to cover government employees’ salaries in San’a during several years of relative quiet, but fighting resumed this summer and quickly escalated. The Houthis seized territory along the Red Sea’s Bab al-Mandeb chokepoint, threatening Saudi oil exports, and repeatedly struck the international airport in Riyadh, leaving at least 18 dead and choking off flights for weeks.

Saudi-backed forces launched a counterattack early this month but have made little decisive progress. The Houthis, meanwhile, are threatening the strategic city of Taiz and repeatedly attacking the seat of government at Aden.

Saudi Arabia will need to either recruit more fighters on the ground or send in more of its own forces into Yemen if it wants to mount a major offensive against the Houthis.

The militia’s latest attacks show how bloody that fight could get. The Houthis now employ far more sophisticated drones, missiles and intelligence. And they are showing they can launch precise and well-timed strikes inside and outside of Yemen.

“This is not 2015,” said Fatima Abo Alasrar, a Yemen expert who writes at The Ideology Machine project, which explores Mideast propaganda and authoritarian governments. “This is exponentially worse.”The al-Alam camp targeted by the Houthis sits at a strategic point in Yemen’s Shabwah governorate, a desert stretch known for being home to the divided country’s oil and natural gas fields.

Houthi attacks stopped energy exports in 2022, but the territory and its economic potential remain crucial to the internationally recognized government’s hold on its part of the country as well as Houthi aspirations to conquer all of Yemen.

Forces allied to the United Arab Emirates, once a partner with Saudi Arabia against the Houthis in the war, held the camp for years before the U.A.E. withdrew from the area in 2021. Saudi-backed fighters have since used the site for training, logistics and commanding forces in the area.

On Oct. 19, hundreds of men who had enlisted into local security forces arrived at the site for training after receiving promises they would be paid to show up, a Yemeni official familiar with the incident said.

Witnesses told The Wall Street Journal that three ballistic missiles struck the base after four battalions massed. One hit recruits as they fled the initial attack, the witnesses said.

Saudi troops were standing ready for the training event when the missiles struck, Yemeni and Saudi officials said. Some Saudis were among those hurt, though officials wouldn’t elaborate.

Witnesses described a chaotic scene as fighters looked nervously to the sky, carrying away severely wounded compatriots without the benefit of air-defense cover.

Relatives of the wounded and dead blamed the local Yemeni commander at the base for failing to respond to warnings about drones that had circled over the site in the days before the attack.

Local authorities in Shabwah demanded that Saudi Arabia expedite the provision of air-defense systems and early warning and monitoring to guard against future strikes. Officials said there were no such systems around al-Alam camp.

The Houthis’ sophistication has grown with the support of Iran. Some Yemeni officials said they suspected the camp had been infiltrated by Houthi informers.

A United Nations report published last year said that the Houthis possess advanced surveillance mechanisms and technology. Their control of telecommunication equipment from San’a allows them to intercept communications through Yemeni telephone numbers, engage in telephone hacking and read and copy messages, the report said.

The week before the attack, Yemen’s government condemned the Houthis’ use of civilian telecommunications networks and services to support military operations and espionage. It also warned the public that spying for the Houthis could lead to charges that carry the death penalty.