Sunni Saudi Arabia and Shiite Iran have been enemies as long as they have existed, but they have largely avoided direct confrontation, facing off instead through proxies they supported in the Middle East.  On Wednesday, when Saudi Arabia launched massive airstrikes against the Iran-backed Houthi rebels in Yemen, that simmering proxy war turned into a conflagration that threatens to engulf the region. The Saudi attack threatens Iran’s control over the country, and turned the poorest Arab country into a battleground between the richest and its historical nemesis.

Saudi Arabia is leading a coalition of at least 10 countries conducting or assisting with strikes in Yemen, in support of the elected Sunni government, whose president has fled the country. Dubbed Operation Decisive Storm, the attacks will include air, ground and naval forces. In response, the Shiite Houthis have vowed a crushing response. And in just 24 hours, the conflict has pulled in the world’s biggest powers, including the U.S., Great Britain and Russia.

"There is an aggression underway on Yemen and we will confront it valiantly," Mohammed al Bukhaiti, a member of the Houthis’ political wing, told Al Jazeera. "Military operations will drag the region to a wide war."

Saudi Arabia reportedly has 150,000 troops mobilized and may send 5,000 soldiers to the southern city of Aden in the coming days. Several reports have quoted Egyptian officials vowing to provide air, ground and naval support to the Saudi-led coalition. The United Arab Emirates provided 30 fighters jets, Bahrain supplied eight, Morocco and Jordan each provided six. For sheer numbers, it’s the largest coalition assembled after World War II that does not feature the direct involvement of U.S. forces. The U.S., however, is participating in the coalition with intelligence, assistance in targeting and logistical support.

Iran has called for an immediate end to "U.S.-backed aggression,” said Foreign Minister Mohammad Javad Zarif. "We have always warned countries from the region and the West to be careful and not enter short-sighted games and not go in the same direction as al Qaeda and Daesh," Zarif said, using the Arabic acronym for the Islamic State group.

As airstrikes resumed Thursday afternoon, reports circulated that Houthi forces were aiming to reach the Saudi border, and eventually cross it, turning the war into a ground conflict  “It’s possible they would consider attacking Saudi territory again” after having occupied portions of it in the past, Alexis Knutsen, a Yemen analyst at the American Enterprise Institute’s Critical Threats Project, said.

When asked if an invasion of Saudi Arabia was part of their strategy, a source with family ties to the Houthis would not confirm or deny.

Iran and the Houthis have consistently denied that their relationship includes direct military support. But since the group seized the capital late last year, there have been several reports that Iran has provided it with weapons, training and funding. Last month, a Houthi delegation visited Tehran and signed a civilian aviation agreement that would facilitate travel between the countries. And unconfirmed reports circulated on Thursday that members of Hezbollah were in Yemen training and arming Houthi fighters for the push toward Aden, the last stronghold of President Abed Rabbo Mansour Hadi.

The Houthis are trying to gain legitimacy in Yemen’s fractured, tribal-dominated politics by concluding economic deals with Russia and China, as well as Iran, and courting public support on the ground by fighting al Qaeda in the Arabian Peninsula. They may also benefit from the airstrikes targeting them. “The airstrikes may unite Yemenis, even those who oppose the Houthis,” Knutsen said.

One victory the Houthis may already have gained is against the U.S.-backed president, who landed in Saudi Arabia on Thursday after leaving Aden. “It’s going to be extremely difficult for Hadi to make any sort of comeback at this point. He’s fled the country,” Knutsen said. “The optics don’t work in his favor right now.”

With Hadi out, Saudi Arabia may be ready to move into Aden before the Houthis do, but it may encounter a hostile population there. Yemen’s south has been the target of U.S. drone strikes targeting al Qaeda for years, and the resulting collateral damage with the killing of civilians has soured many locals against America and its allies. The Saudi airstrikes have made that problem worse, with the death of at least 17 civilians since the airstrikes began. Given the option between airstrikes and foreign intervention, or the Houthis, some may choose the local option.

In the meantime, Houthi leaders are taking a fighting stance that may signal their willingness to take the fight to Saudi Arabia.

“We don't need Iran,” Hussain al Bukhaiti, a pro-Houthi translator and journalist in Sanaa and Mohammed al Bukhaiti’s brother. “The problem with the Gulf and Western countries [is that] they underestimate Houthi strength. So now the whole world will know who the Houthis are.”