More than four years of war have claimed tens of thousands of lives and turned Yemen into the world’s worst humanitarian disaster.
On one side of the conflict is the government-in-exile, backed by a Saudi-led coalition of Arab states and the U.S.
On the other is a rebel force with a much lower international profile: the Houthis.
The group, believed to include at least 100,000 fighters, is supported by Iran and controls about 20% of the country, including the north and the capital, Sanaa.
Preliminary peace talks in Sweden this month, along with legislation advanced in Congress to end U.S. support for the war, have raised hopes for ending the war.
But the goals of the Houthis remain somewhat murky. Here are some facts about the group:
Who are the Houthis?
The Houthis are a political and religious movement that grew out of efforts in the early 1990s to stop persecution of Zaidi Muslims.
The faith, an offshoot of Shia Islam, is nearly exclusive to northern Yemen and is practiced by about a third of the country’s 28 million people.
The Zaidis were once royalty. For more than 1,000 years they ruled a state — called an imamate — that covered what is now northern Yemen. But in 1962, with anti-royalist and socialist movements sweeping the region, military officers overthrew the Zaidi king and declared North Yemen a republic.
An eight-year civil war ensued, and when the junta finally prevailed, the government it set up made a point of marginalizing the Zaidis.
The aim was to prevent them from returning to power. Ali Bukhaiti, a former political spokesman for the Houthis, said all references of Zaidis were excised from textbooks.
“The state felt the Zaidi creed was a threat,” he said. “It didn’t want any mention of an ‘imamate.’ ”
With help from Saudi Arabia, the government also blunted the power of Zaidis by moving Salafist and Wahhabist Muslims — two austere Sunni sects — to the Zaidi-dominated north to be school teachers.
North Yemen and South Yemen were separate countries until being united in 1990. The new president, Ali Abdullah Saleh, was a Zaidi. But that only went so far in reassuring other Zaidis that their rights would be protected.
Some joined the Believing Youth, a new summer camp program with a mission of promoting Zaidi Islam in the north while preaching tolerance for other forms of the the religion.
Others entered electoral politics — most notably a young cleric named Hussein Badreddin Houthi.
After two failed bids to enter parliament in 1993 and 1997, he joined the Believing Youth and eventually came to lead the group, which had grown to educate tens of thousands of students.
They went on to form the backbone of the movement that would be later be named for the cleric.
How did the movement turn violent?
It was 9/11 and the U.S. invasion of Iraq that put Houthi on a collision course with the government.
Saleh opened Yemen’s skies and military bases to the U.S. That infuriated Houthi and his followers, who advocated a strong line against the West. In his defiance, he introduced the sarkha, or scream, that would come to define the Houthi movement: “Death to America! Death to Israel! Damn the Jews! Victory to Islam!”
In 2004, Saleh accused Houthi of numerous crimes — setting up unlicensed religious centers, seeking to restore the imamate and being part of a foreign terror network — and launched what was supposed to be a simple arrest operation.
But Houthi resisted, and his adherents rose to protect him. Government forces trapped and killed him later that year, turning him into a martyr and sparking an insurgency led by his father and brother — Badreddin Houthi and Abdul Malek Houthi.
Over the next few years, the ragtag force, which hid in caves and was armed with little more than AK-47s, came to be known as the Houthis.
Government forces, funded by the Saudis and supplied with U.S. arms, jets and tanks, fought the insurgents with scorched-earth tactics that included mass arrests and sieges on areas they held.
In 2010, a ceasefire halted six years of fighting and raised hopes for peace.
At the time, the Houthis claimed to have 100,000 fighters. Stephen Seche, a former U.S. ambassador to Yemen, said many of them were hardened veterans who were primed for more war.
“They’re just good at it, they’re experienced, and for them it’s almost a way of life,” he said.
How did the Houthis come to rule Sanaa?
During the pro-democracy “Arab Spring” revolutions of 2011, the Houthis joined in widespread calls for Saleh’s ouster.
After he left, they entered talks with the new government of President Abdu Rabu Mansour Hadi, negotiating an agreement to protect their rights and end their marginalization.
“They were supposed to be integrated in the state, with positions in government, even the army,” said Bukhaiti, who represented the Houthis in the negotiations .
But the accords were never fully implemented, and the Houthis began to consolidate their grip over the country’s north. Soon they had their sights on Sanaa, where Hadi was facing increasing anger over fuel price hikes and cronyism.
The Houthis turned to their old nemesis Saleh, who was no longer in power but still had the loyalty of much of the army and was willing to strike a deal with the hope that it would return him to power.
Working alongside Republican Guard units loyal to Saleh, the Houthis stormed Sanaa in September 2014, receiving a celebratory welcome.
Hadi escaped to Aden and from there to Saudi Arabia, where he remains in Riyadh as the head of the internationally recognized government.
In early 2015, the Houthis made a play for the city of Aden. That spurred a fierce reaction from Saudi Arabia and the United Arab Emirates, which formed a coalition and with U.S. assistance began a brutal air campaign on Houthi-held areas.
Thousands of people were killed as the war continued to ramp up. The coalition also established a full land and sea blockade that brought the country to the brink of famine.
What about Iranian support?
After the initial insurgency in 2004, Saleh insisted that the rebels were receiving assistance from Iran and solicited more help from the U.S. to stop what he described as the tip of the Iranian spear.
U.S. officials, according to cables published by Wikileaks, didn’t appear to believe him.
But by 2011, it was clear Iran was helping the Houthis to counter the growing power of Saudi Arabia in Yemen.
Bukhaiti said Iran would sell tankers of oil and have brokers deliver the proceeds, along with arms, to the Houthis. At the same time, Hezbollah, the Lebanese political party and armed group that is seen as Iran’s most successful guerrilla force, began hosting Houthi officials and helped open Al Masirah, a Houthi broadcaster, in the suburbs of Beirut.
Bukhaiti downplayed the influence that Iran has over the Houthis and the importance of outside help.
“The Houthis have more experience in fighting than Hezbollah,” he said.
Will the Houthis accept peace initiatives?
Preliminary negotiations at a picturesque castle in Sweden this month resulted in ceasefires and fledgling steps toward resolving the conflict. More talks are scheduled in January.
The U.S. Congress, reacting to the recent killing of Saudi journalist Jamal Khashoggi by his own government, is pushing the Trump administration to withdraw support for the war effort.
Still, few observers believe the group is engaging in the negotiations in good faith, even if the other side manages to fulfill its end of the deal.
“The Houthis were unjustly treated, but now they have become out of control,” Bukhaiti said. “There’s no way to get to them unless you break them militarily.”