FOR MONTHS Saudi Arabia has talked up a possible new offensive against the Houthis, the Iran-backed rebels who control much of Yemen. The kingdom’s Yemeni allies promised they had “surprises” in store. Propagandists blustered that they might soon free Hodeida, the country’s main port, and perhaps even Sana’a, the capital.When the fight came, however, it was the Houthis who delivered a shock. Over the past few days, their forces have advanced swiftly down Yemen’s western coast. (REUTERS)When the fight came, however, it was the Houthis who delivered a shock. Over the past few days their forces have advanced swiftly down Yemen’s western coast. On Thursday they captured Mokha, a port city (and the namesake of the coffee bean). Then they seized Perim island, a rocky outcrop in the middle of Bab al-Mandab, the strait that links the Red Sea to the Indian Ocean.At the same time, drones hit a major oil pipeline in Saudi Arabia, which carries up to 7m barrels per day (b/d) to the kingdom’s Red Sea ports. It was shut down “as a precautionary measure”. The Saudi foreign ministry blamed the attack on Iraqi militias, although the details are murky.The Houthi advance is not only an embarrassment for Saudi Arabia. It also gives Iran new leverage in its war with America: its allies have bolstered their grip on another vital waterway. Brent crude surged as high as $109 a barrel on September 11th (though it ended the day slightly lower). The Saudis will not want to accept Houthi control of the coast, but pushing them back will be difficult.Yemen’s civil war began in 2014, when the Houthis swept out of the northern highlands and seized major cities. A Saudi-led coalition invaded the following year. The war devolved into a stalemate, and the front lines had been largely frozen since a 2022 ceasefire—until America’s attack on Iran upended the status quo.Iran has spent months pressuring the Houthis to join the fray. In July Saudi jets bombed the airport in Sana’a to prevent the arrival of an Iranian plane carrying military advisers from the Islamic Revolutionary Guard Corps. Days later, the Houthis announced a blockade on Saudi shipping through the Red Sea. They also resumed missile attacks on Saudi cities, which had been halted for several years. A return to serious fighting looked inevitable, and it was obvious that the Houthis would try to seize the coast. Yet when they did, the Saudis and their allies looked unprepared.The anti-Houthi factions are a disorganised bunch: they include religious fundamentalists, separatists who seek an independent south Yemen and tribal militias, to name but a few. Some are aligned with Saudi Arabia, others with the United Arab Emirates (UAE). The latter were regarded as the more effective. But the Saudis forced the Emiratis to withdraw from Yemen in December, after UAE-backed separatists seized territory and made noises about declaring their own state.That left a schism within the opposition. As the Houthis advanced, many of their foes simply fled. Rival commanders have spent the past few days accusing one another of betrayal. Retreating fighters sold their weapons by roadsides. Spin doctors in Riyadh gamely insist this was all a cunning ploy to lure Houthi fighters into a “kill box”, where Saudi jets could bomb them. But Yemeni sources complain that the Saudis have been slow to provide air support.More sober Saudi voices now argue that the kingdom needs foreign support. “The question quickly becomes whether outside powers can afford to remain spectators,” says Ali Shihabi, a commentator close to the royal court. Yet it is unclear who might provide it. This is the first test of the so-called Mecca alliance, a mutual-defence pact between Saudi Arabia, Turkey and Pakistan announced to great fanfare last month. So far the kingdom’s new allies have sent nothing beyond well wishes.America has deployed advisers to help the Saudis with targeting and intelligence, but Donald Trump declined to launch his own air strikes despite personal entreaties from Muhammad bin Salman, the Saudi crown prince and the kingdom’s de facto ruler. “The Houthis called us, and they don’t want to fight,” the American president said on Saturday. Even if he changes his mind, America’s capabilities are strained. A prolonged campaign would probably require diverting one of two aircraft-carrier groups stationed near the Strait of Hormuz. That would undermine America’s blockade of Iranian ports, as well as its effort to escort oil tankers through the waterway, which has had some success of late.The war in Yemen can be fluid: the Houthis have made big advances before, only to find themselves overextended. They are likely to move next on Taiz, a mountainous city that was besieged for much of the war, and Marib, the hub of Yemen’s oil-and-gas industry. Air power alone will not be enough to push them back, however. Time will tell whether the Saudis can mount a serious counter-offensive.In the meantime, oil prices will probably head up. Even before the latest offensive, Houthi threats were scaring vessels away from Bab al-Mandab: Saudi Arabia exported just 200,000 b/d through the strait in August, down from 4m b/d in June, according to Kpler, a data firm. Saudi Aramco, the state-run oil giant, can probably repair the damaged pipeline quickly. A pumping station hit by an Iranian drone in April was fixed within days. But the risk of further attacks will remain.