An attack on a major Saudi oil facility on Saturday knocked out 5% of the global supply, and roiled the markets.

What's more, it was achieved with easy-to-acquire and relatively cheap military equipment.

Saudi Arabia has spent billions defending its oil fields, but this came to nothing.

The attacks appear to be designed to show how easily such an attack can threaten the world economy.

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The drone and cruise missile attack on Saudi oil production facilities sent an unmistakable message: even low-tech weaponry like that used by Iran and its allies in Yemen, the Houthis, can easily overcome billions of dollars of air defenses.

The strike, and the chaos it inflicted on global oil markets, dramatically put the region on notice that any expanded military conflict in the Gulf will badly punish both sides.

US officials have claimed direct Iranian involvement in the attack, the first large-scale use of cheap guided munitions on a Saudi oil facility. Iran has denied responsibility.

This image provided on Sunday, Sept. 15, 2019, by the U.S. government and DigitalGlobe and annotated by the source, shows a pre-strike overview at Saudi Aramco's Khurais oil field in Buqyaq, Saudi Arabia. U.S. government/Digital Globe via AP

The strike has proved so effective because it struck a key production bottleneck, knocking out in one go around 5% of the world's oil production.

A former CIA officer, Bob Baer, who is now an author and pundit, has long been warning about the vulnerability of the oil processing facility at Abqaiq. It is vulnerable, he says, because it is a bottleneck through which much of Saudi's oil must pass before being exported.

The facility, spread over hundreds of acres, is filled with very specific and hard-to-replace custom equipment.

A composite image of two satellite photographs which show areas of Saudi oil facilities in Abqaiq and Khurais. AP/Business Insider

In his book, "Sleeping with the Enemy," Baer calls Abqaiq "the Godzilla of oil-processing facilities." He predicted — seemingly correctly — that a successful attack could knock offline about 5 million barrels per day of production.

On Sunday, Saudi officials said that 5 million barrels per day had in fact gone offline. They promised that 2 million barrels could be restored on Monday.

While rebutting claims it was responsible, Iran instead it presented the attack as a logical response to Saudi's five year bombing campaign in Yemen. (Iran-backed Houthi rebels have said the attack was their doing.)

The Iranian response appears designed to reduce pressure on the US to expand the escalating military conflict, while also making the point that the entire world would suffer painful oil shortages if wider war were to break out.

Saudi Arabia and its mercurial crown prince, Mohammed bin Salman, entered the Yemeni civil war five years ago and since have conducted massive airstrikes on Houthi targets across Yemen that have terrorized the population while doing very little to degrade Houthi abilities or deter Iran from supporting its Yemeni allies.

Shi'ite Muslim rebels hold up their weapons during a rally against Saudi air strikes in Sanaa, Yemen, March 26, 2015. REUTERS/Khaled Abdullah

By Monday morning this step to hit Saudi oil processing facilities appeared to contain significant risk of that very conflict expanding, as shown by President Donald Trump's use of the phrase "locked and loaded" to describe the US thinking on a military response.

But with 5% of the world's oil production knocked offline by what appeared to be an attack by cheap, easy-to-operate drones, and Iranian produced knock off Chinese cruise missiles, it was clear throughout the region that the billions that Saudi Arabia has spent on air defenses had done nothing to stop a relatively cheap offensive technology.

The Houthis immediately claimed the attack and Iran denied any special involvement in the attack. US and Saudi officials have claimed, without hard evidence, that the attacks might have been launched from southern Iraq, where Iran has significant capabilities via the militias they support.

Yemen's President Abed Rabbo Mansour Hadi, left, walks with then-Saudi Defense Minister Mohammed bin Salman as he arrives at an airbase in Riyadh, Saudi Arabia, Thursday, March 26, 2015. AP Evidence available is inconclusive as the weapons used appear to have enough range to have come from Yemen, Iraq, or even Iran itself, making their origin impossible to guess without better information.

"At a minimum Iran provided these weapons if not direct involvement. And For the Houthis there is probably a benefit to claiming these attacks even if it's wasn't just them," tweeted Ilan Goldberg, director of Middle East affairs for the Center for a New American Security, and a former US government official for the region.

"Makes them seem more fierce and credible & the Saudis are already at war with them so the risk of a major Saudi response beyond the current fight is relatively low."

The attack came at a sensitive time more broadly: US-Iran relations were in flux after anti-Iran hardliner John Bolton was fired as Trump's national security adviser. US officials had floated the possibility of Trump meeting Iranian President Hassan Rouhani soon — a prospect which now seems distant.

The attack seems best interpreted as a warning to the US, that continued violence in the region could bring more disruption, and roil the global economy as Trump prepares for a 2020 election against a backdrop of flagging US performance.