The Saudi-led—and U.S.-backed—war in Yemen turns two on March 26. The war, initially dubbed “Operation Decisive Storm,” and now more ironically called “Operation Restoring Hope,” has failed to achieve any of its aims. Yemen’s Houthi rebels, who Saudi Arabia erroneously claims are Iranian proxies, have retained control of northwest Yemen and Yemen’s unpopular government remains in exile. In addition, the war has succeeded in impoverishing millions, empowering al-Qaeda in the Arabian Peninsula (AQAP), and now threatens the broader region with increased instability.

Yemen, already the Middle East’s poorest country, now faces a catastrophic famine. Sixty percent of Yemenis are at risk of starvation. More than 17 million people out of the population of 24 million are in need of immediate aid. Two years of often indiscriminate bombing by Saudi Arabia and its primary partner, the United Arab Emirates (UAE), have laid waste to Yemen’s infrastructure.

Saudi planes—which are dependent on mid-air refueling capabilities provided by the U.S.—have targeted bridges, roads, factories, hospitals, farms, and even funerals. Unexploded cluster munitions used by Saudi Arabia litter some of Yemen’s most productive agricultural land where they will kill and maim for years to come. More than 10,000 people—mainly civilians—have died in the war. Thousands—possibly tens of thousands—more will die from starvation and disease in the months to come.

Saudi Arabia’s inability to achieve its aims in Yemen despite an aerial campaign that appears to know few ethical constraints is telling on two fronts: first the war has shown Saudi Arabia’s military to be a paper tiger. Second, the war has again shown the very real limits of advanced weaponry when faced with a creative, resolute enemy who knows how to leverage complex terrain.

Despite spending billions of dollars on the latest weaponry—in 2016, Saudi Arabia had the third largest military budget in the world—it is unable to defeat a poor but determined foe. The Kingdom’s defense budget has ballooned since the revolutions that swept through the Middle East in 2011. It has lavished money on its air force and army, the Royal Saudi Land Forces and the Saudi National Guard. The spending spree has been a gift to U.S.- and British-based arms manufacturers.

For most of the last eight years, the U.S. Congress and the Obama administration have readily approved lucrative arms sales to Saudi Arabia. Toward the end of the Obama administration, a temporary halt was placed on the sale of precision guided munitions to Saudi Arabia. This was the only significant sign that Saudi Arabia’s unrestrained bombing of Yemen might have given some U.S. lawmakers pause. However, the Trump administration, via the State Department, has indicated that it will resume arms sales to Saudi Arabia.

This follows Saudi Deputy Crown Prince Muhammad bin Salman’s visit to the White House on March 14. The young deputy crown prince is also Saudi Arabia’s defense minister and is a key backer of his country’s war in Yemen. Given the Trump Administration’s paranoia about Iran, King Muhammad bin Salman and the Saudi government likely secured further promises of support for their war in Yemen.

While the arms sales will no doubt make billions more for U.S. arms manufacturers, the additional weapons are unlikely to help Saudi Arabia win its war in Yemen. In fact, Saudi Arabia’s Royal Saudi Land Forces (RSLF) and its national guard have proved incapable of defending the country’s southern border against repeated retaliatory incursions by the Houthis and Yemeni Army units allied with them. Despite spending $87 billion on its military in 2016, Saudi Arabia recently requested that the Pakistan Army send units to help it defend itself against the sandal clad Houthis who have no air support and who most often fight with small arms.

Even with the help of mercenaries (several thousand mercenaries are deployed in Yemen) and advanced weaponry, Saudi Arabia is unlikely to achieve its aims in Yemen. Yemen has a long history of bedeviling invaders that dates back to the Romans, who sent an army under the leadership of Aelius Gallus to invade so that they could control the incense trade. Gallus’s army was routed in Marib despite its superior war fighting technology and numbers.

More recently, Yemen proved to be a graveyard for a generation of Ottoman Turks and Egyptians. The Ottomans struggled to control the country and, ultimately, gave up. The Egyptians intervened on the side of Republican forces in North Yemen’s 1962–70 civil war. Their intervention cost them at least 25,000 dead soldiers.

The Egyptians had what—for that time—was advanced weaponry. They also fought with few ethical constraints. The Egyptians even used chemical weapons on what were then Saudi backed Royalist forces—ironically the grandfathers of many of the men the Saudis are now fighting—to no avail. The Egyptians were defeated by men who fought with 60-year-old Lee Enfield rifles and had no air support and limited supplies. The Egyptians—like the Turks before them—learned that superior forces and advanced weaponry do not easily overcome a determined enemy fighting in what are some of the most rugged mountains in the world.

Yemen’s mountains, along with the tough and resilient men and women that they have forged, are its greatest asset when it comes to thwarting invaders. In 1967, the English explorer and Arabist, Wilfred Patrick Thesiger, visited Royalist forces that were holed up in the mountains of northwest Yemen. The veteran explorer of the Hindu Kush mountains and the Empty Quarter section of the Arabian Desert said, “nowhere have I experienced more strenuous traveling than in the Yemen.”

Just as in Afghanistan—which is not dissimilar to Yemen in terms of its geography and tribal politics—mountains are an almost magical force multiplier. If the Saudis want a picture of their future in Yemen, they need only look at the United States’ war in Afghanistan, now the longest foreign war in U.S. history. Despite spending well over a trillion dollars and utilizing the most advanced weaponry available, the U.S. has failed to defeat the Taliban. The U.S.-backed Afghan government controls only 63 percent of the country, with steady losses to the Taliban over the last two years. If the U.S. cannot win in Afghanistan with what is the best equipped and trained military in existence, how can the Saudis possibly expect to prevail in Yemen with a military that cannot even defend its southern border?

The simple answer is that they will not prevail. The war in Yemen will grind on—at least at a low level—for years to come if the U.S. continues to enable Saudi Arabia. All the while, millions of Yemenis will continue to suffer and die and groups like AQAP will flourish. AQAP has already benefited immensely from the war and from Saudi Arabia’s willingness to turn a blind eye to a group that is fighting the same enemy, the Houthis. Most critically, going forward, AQAP will be able recruit from an entire generation of Yemenis who will be far poorer and less well-educated than their parents.

The blowback from the Saudi-led war in Yemen will spread well beyond Yemen’s borders. It already has. Retaliatory raids by the Houthis deep into the Saudi provinces of Jizan, ‘Asir, and Najran have inflamed tensions in those provinces where the Saudi government and its hardline Wahhabi version of Islam have never been popular with much of the population which includes Zaidi and Ismaili Shia. Further afield the weapons that both Saudi Arabia and the United Arab Emirates have poured into Yemen—ostensibly to arm militias to fight the Houthis—are being smuggled to places like Somalia. Somalia’s own al-Qaeda affiliate al-Shabaab has long suffered from a lack of advanced weaponry. It is all but certain that al-Shabaab along with Somalia’s pirate gangs will benefit from the influx of arms to the country.

As the Saudi-led war in Yemen turns two, one can only hope that the Trump administration will not double down on the Obama administration’s policy of largely giving the Saudis carte blanche in Yemen. Such a policy does not serve the interests of U.S. national security and it does nothing to contribute to regional stability. Rather, it will do quite the opposite.

The 1962–70 civil war in what was then North Yemen that drew in the Egyptians and other outside powers, including Saudi Arabia, was finally settled after those powers withdrew their troops and support. Then Yemenis were able to draw on their long history of negotiated settlements and end the war. Such a process might begin again if the U.S. were to use its still considerable influence to restrain Saudi Arabia. Unfortunately, there are billions of reasons why this is unlikely to happen.

Michael Horton is a senior analyst for Arabian affairs at the Jamestown Foundation. He is a frequent contributor to Jane’s Intelligence Review and has written for numerous other publications including: The National Interest, The Economist, and West Point’s CTC Sentinel.