A day after Saudi Arabia and a coalition of nine other states began hammering the Houthis with airstrikes and blockading the Yemeni coast, President Abdel-Fattah el-Sissi of Egypt said in a statement that the country’s navy and air force had joined the campaign and that its army was ready to send ground troops “if necessary.”

The Associated Press, citing unnamed military officials, reported that plans for an Egyptian invasion were already underway, and many analysts had already concluded that airstrikes alone had little chance of pushing back the Houthis.

Saudi news media declared that the offensive, which began Wednesday night, had fully disabled the Houthi-aligned Yemeni air force.

Iran, Saudi Arabia’s regional rival and the Houthis’ main ally, denounced the assault as a U.S.-backed attempt “to foment civil war in Yemen or disintegrate the country.” Houthi-controlled television channels broadcast footage of dead bodies and wounded civilians, blaming “American-backed aggression.”

‘Face this aggression’

The movement’s leaders warned that the battle could widen into a regional conflict, but they also vowed to overcome the Saudi attacks without Iranian help. “The Yemeni people are prepared to face this aggression without any foreign interference,” Mohammed al-Bukhaiti, a Houthi spokesman, told Reuters.

Prices for crude oil rose about 4 percent Thursday over concerns that the fighting in Yemen might affect the passage of tankers through the Bab el Mandeb strait, a narrow chokepoint between Yemen and Africa that is the entrance to the Red Sea.

Along with Iraq, Libya and Syria, Yemen is the fourth Arab nation where an attempt to build a new democracy has been consumed by civil conflict, regional proxy wars and the expansion of extremist groups like the Islamic State in Ira and the Levant and Al-Qaida.

The Houthi leadership, which hails from northern Yemen, practices a variant of Shiite Islam, the religion of the Iranian theocracy. Saudi Arabia, the region’s Sunni Muslim power, is backing forces loyal to President Abed Rabbo Mansour Hadi, who has fled the country.

The Iranian president, Hassan Rowhani, criticized the airstrikes and said “regional countries should restrain from any actions which may intensify the crisis,” the presidential website reported. Rowhani added, “We should make extensive efforts for three aims: stopping military interference in Yemen, preventing any opportunities for terrorists to take control and helping setting up a sovereign government.”

Strongman’s influence

The Saudi Arabian-led military intervention immediately raised the threat that Iran might retaliate by increasing its own support for the Houthis with money and weapons — as Tehran has in the past — or with a more active military role, escalating the violence. But the struggle for Yemen is more than merely a sectarian conflict or a regional proxy war, partly because of the singular role of Ali Abdullah Saleh, the country’s former strongman.

Saleh left power under pressure from an Arab Spring uprising under a transitional plan brokered by Saudi Arabia and the other Persian Gulf states. As president, he fought wars against the Houthis and at times appeared to ally with Saudis against Iran.

But he is a member of the same Shiite sect as the Houthis, and he has now struck an alliance with them in an apparent bid to restore his power. He has helped lead units of the Yemeni military and security services to swing to the side of the Houthis against his successor, Hadi, and analysts say Saleh has played a much more critical role than Iran has in enabling the Houthi advance.

Some of the Houthi allies have even begun calling for the election of the former president’s eldest son, Ahmed Ali Abdullah Saleh, as Yemen’s next leader. An ambassador to the United Arab Emirates and a former commander of Yemen’s elite republican guard, the face of the younger Saleh now appears on billboards around Houthi-controlled Sana urging his selection as the country’s next president.

The United States and most Arab nations moved quickly to support the Saudi-led operation in Yemen, which Saudi Arabia has called Operation Decisive Storm.

The White House said in a statement that the United States would provide “logistical and intelligence support” to the Saudi-led military operations.