Smoke is seen following a fire at Aramco facility in the eastern city of Abqaiq, Saudi Arabia, September 14, 2019. Stringer | Stringer

DUBAI — Questions have abounded all week as to how Saudi Arabia, the planet's third-highest defense spender and steward of the world's largest oil facility, allowed itself to fall victim to a drone and missile attack that wiped out half of its crude production in a day. "The Saudi leadership has a great deal of explaining to do that a country that ranks third in terms of total defense spending ... was not able to defend its most critical oil facility from these kinds of attacks," former U.S. diplomat Gary Grappo told CNBC on Tuesday. The stakes for the future of Saudi Arabia's ability to defend itself are global. Brent crude saw its largest price jump ever as markets opened this week, and the commodity's next moves depend heavily on Saudi oil giant Aramco's ability to recover its production capacity and defend itself from similar attacks.

Investors are likely asking themselves how the kingdom could have left itself so vulnerable and what that means for the future of oil, global markets and the long-awaited Aramco public stock offering. So how did the Saudis, who in 2018 spent an estimated $67.6 billion on arms — second only to the U.S. and China — fail to defend their economic jugular vein?

A target like 'a Christmas tree'

Quite simply, the kingdom's defenses — no matter how high-tech — are designed for entirely different kinds of threats. The low-flying and relatively cheap drones and cruise missiles purported to have been used in Saturday's attack are a fairly new challenge that many nation states are not in fact prepared to counter.

The Saudis have a lot of sophisticated air defense equipment. Given their general conduct of operations in Yemen, it is highly unlikely that their soldiers know how to use it. Jack Watling Land warfare expert, Royal United Services Institute

It also doesn't help that massive oil plants are just easy targets. "Saudi oil assets are vulnerable for the simple reason that when flying over them at night, they stick out against the desert background like a Christmas tree," Michael Rubin, a former Pentagon official and Middle East expert at the American Enterprise Institute, told CNBC in an email. "This means that enemies don't need high-tech GPS-guided drones, even though they might have them, but can also use relatively lower technology drones."

Drone wreckage including one described as an Iranian Delta Wave UAV, foreground, from the attack on the Aramco Abqaiq oil refinery, sits on display during a Ministry of Defense news conference in Riyadh, Saudi Arabia, on Wednesday, Sept. 18, 2019. Saudi Arabia on Wednesday said the weekend attacks on the kingdoms critical oil infrastructure were "unquestionably sponsored by Iran." Photographer: Vivian Nereim/Bloomberg via Getty Images Vivian Nereim | Bloomberg | Getty Images

Twenty-five drones and missiles were used in the Saturday strikes on state oil giant Saudi Aramco facilities Abqaiq and Khurais, Saudi's defense ministry said. While claimed by Yemen's Houthi rebels, Saudi and U.S. officials say Iran was responsible, a charge Tehran has denied. Dave DesRoches, an associate professor and senior military fellow at the National Defense University in Washington, D.C., told CNBC: "If an attack is of a different threat than the system was designed for — that is a low-altitude cruise missile instead of a high-altitude ballistic missile — then the system will not intercept it."

Saudi Arabia's current air defenses are 'irrelevant'

Saudi Arabia boasts an arsenal of sophisticated and expensive air defense equipment. They have the American-made Patriot missile defense system, German-made Skyguard air defense cannons and France's Shahine mobile anti-aircraft system, and they'll soon have Lockheed Martin's highly advanced THAAD (terminal high altitude area defense) interceptors. But these are basically inconsequential, says Jack Watling, a land warfare expert at the Royal United Services Institute who advises Gulf militaries. "The Patriots are kind of irrelevant," Watling told CNBC in a phone interview. "The track record of Patriot engaging missiles of any kind is pretty awful, they very rarely hit the target." The other issue, he says, is that it's designed for shooting down high-altitude ballistic missiles, not the cruise missiles and drones used in Saturday's attack. "These were low-flying cruise missiles. They were coming in far below the engagement zone for Patriot. So you wouldn't have tried to hit them with Patriot." In its primary role of shooting down aircraft, Watling noted, the system does perform "very well." Tweet1 Aerial photographs found on open-source platforms show three Skyguard batteries placed around the targeted Abqaiq oil facility, which are slow-firing large caliber anti-aircraft guns, as well as French-made Shahine batteries from the 1980s. Despite being meant to protect these facilities, they Skyguards were not of much use either, Watling says: "The batteries around the site are firstly not the appropriate systems to engage cruise missiles, and there is no evidence that the Saudis have trained using their equipment."

'The Saudis… are largely inattentive'

To add to the Saudis' weapon woes, their military personnel may not be up to the task either, according to Watling and several other experts who spoke to CNBC anonymously. "The Saudis have a lot of sophisticated air defense equipment. Given their general conduct of operations in Yemen, it is highly unlikely that their soldiers know how to use it," Watling said. He added that the kingdom's forces have "low readiness, low competence, and are largely inattentive." "So if you're a battery commander protecting against an oilfield which you never believed was going to come under attack, how carefully are you watching your radar? I'd be surprised if they'd even turned their radar on." Even those that do have the technical knowledge, Watling added, "are unlikely to be attentive enough to pick up small unmanned aerial vehicles or low flying missiles on their radar... quickly enough to coordinate countermeasures." Tweet2 The Saudi Defense Ministry did not respond to a CNBC request for comment. In the Saudi military's defense, oil infrastructure in the kingdom falls under the Ministry of Interior (MOI), not the military, noted Becca Wasser, a security analyst and war gaming expert at RAND Corp in Washington, D.C. "Most of U.S. arms sales to KSA, particularly in air defense, have been to the military," she wrote on Twitter on Monday. "The MOI, to my knowledge, isn't well kitted out for this role as they tend to focus on domestic threats."

So what does the kingdom need to do?