The turmoil that has raged for months inside Yemen, an impoverished country at the bottom of the Arabian Peninsula, exploded this week into a dangerous and growing regional power struggle.
Jets from Saudi Arabia bombed military installations in Yemen on Thursday to weaken the Shiite rebels who chased the president out of the country. And there were suggestions of a ground fight to come.
Analysts see a high-stakes shadow struggle between Saudi Arabia and Iran. Meanwhile, American officials are increasingly alarmed, partly because Yemen is central to U.S. counterterrorism operations.
Here’s what you need to know.
The Saudis, joined by Arab allies, bombed the airport in the Yemeni capital of Sanaa and an air base. The Saudi defense minister said that his country is prepared to use ground forces if necessary, and officials in Egypt told The Associated Press that they were prepared to help.
A security adviser to the Saudi kingdom told NBC News that Saudi Arabia was in "complete control" of Yemeni airspace, and that Saudi Arabia had mobilized 150,000 troops along the border.
Iran, which backs the rebels and is a rival of Saudi Arabia, denounced the strikes as an "invasion" that will worsen the crisis in Yemen.
The United States said that it was giving the Saudis military and intelligence support but not taking part in the strikes. Secretary of State John Kerry commended Saudi Arabia.
Asked to explain the American support for Saudi Arabia’s intervention, Jeff Rathke, a State Department spokesman, said: "Really this is a situation they view with concern. It’s also a situation that the United States views with concern."
The rebels, known as Houthis, seized the Yemeni capital in September and forced the American-backed president, Abd-Rabbu Mansour Hadi, to flee. He was in the port city of Aden until Wednesday or Thursday, when he left the country entirely, according to his spokesman. The spokesman said Thursday that Hadi has arrived in the Saudi capital of Riyadh.
Hadi was an ally of the United States in its fight against al Qaeda in the Arabian Peninsula, considered the most vicious branch of the terror network. The United States pulled out 125 military advisers last week as the chaos grew.
The Houthis see Hadi as a puppet of Sunni Gulf states and the West. They are supported by Ali Abdullah Saleh, who controlled Yemen for three decades until he was ousted in 2012.
Saudi Arabia vs. Iran
The conflict amounts to a proxy fight between Sunni-ruled Saudi Arabia and Shiite-ruled Iran.
The Saudis think that keeping Iran in check is critical to their own security, and they see Yemen as "the tip of the Iranian spear," said John Jenkins, a former British ambassador to the kingdom.
"This is the first example of the Saudis pushing back physically against a Shia insurgency and by extension Iran," said Jenkins, who is now the Middle East executive director for the International Institute for Strategic Studies.
Saudi Arabia is threatened by Iranian actions throughout the region, whether it be fighting ISIS in Iraq and Syria or supporting the Lebanese militant group and political party Hezbollah, he said.
"Yemen is the fourth Arab country in which they think the Iranians have established not just influence but a strategic presence," Jenkins said.
Yossi Mekelberg, a professor of international relations at Regents University in London and a fellow at the Chatham House think tank, said that the Saudis also see the United States as "a little bit too close to Iran."
The United States is taking part in negotiations on the future of Iran’s nuclear program, and both the United States and Iranian-backed militias are fighting ISIS in Iraq — though the U.S. says they are not coordinating or cooperating with the militias.
Speaking of the mobilization of the 150,000 troops, Mekelberg said: "You don’t amass this kind of troops if you don’t see it as a real threat to your national interest, to your security and your competition for hegemony in the region."
Who’s Helped by the Chaos?
Al Qaeda and potentially its rival ISIS could benefit, according to a former U.S. ambassador to Yemen and a leading security analyst. Speaking before the Saudi airstrikes, both said the extremists could find more room to operate as the Yemeni government and rebels battle for control.
"It’s going to benefit certainly al Qaeda in the Arabian Peninsula — probably the most dangerous faction of AQ,” said Anthony Cordesman of the Center for Strategic and International Studies. "We don’t know as yet whether there is real support for ISIS."
The withdrawal of the American advisers from Yemen was also seen as a major setback for counterterrorism operations.
"We’re pretty blind now," the former U.S. ambassador told NBC News. "It doesn’t mean the air campaign will stop, but it means that the intelligence and coordination won’t be as good, which leads to more collateral damage, which in turn feeds into the narrative that all the U.S. wants to do is kill Muslims," he said.
Without the advisers on the ground, Washington will also struggle to keep track of Yemen’s rapidly shifting political landscape.
"We lost contact with Yemen forces who remain loyal to central government," Cordesman said. "And we lost the ability to have the kind of human relationships so they know to trust us."
Growing List of Failed States
The number of failed states where ISIS and al Qaeda-linked groups operate in the Middle East is growing to include Iraq, Syria, Libya and now Yemen. The wars in those nations have also driven millions from their homes, creating misery and fertile recruiting grounds of radical groups.
It’s a stark contrast to the Middle East of just over a decade ago, when Iraq, Syria and Libya were secular dictatorships and Yemen had a weak, U.S.-allied government.
The collapse of Yemen is particularly worrisome to U.S. officials because there is already a well-established al Qaeda operation in the country. If Yemen descends further into chaos, analysts warn, al Qaeda could get what ISIS has now: a haven.
"Al Qaeda had already been operating in Yemen after much of it was driven out of Saudi Arabia," Cordesman said. If Yemen fails completely, "it potentially gives (al Qaeda) the possibility of having the same kind of area under their control that ISIS has in Syria and Iraq."