A supporter of the Shiite Houthi movement wears a headband with a photo of the movement's leader Abdulmalik al-Houthi during an anti-government demonstration in Sanaa in this Sept. 3, 2014 file photo. The headband reads, "The people's revolution, a revolution against corruption." (Khaled Abdullah/Reuters)

On Thursday, the entire government of Yemen's president, Abed Rabbo Mansour Hadi – a key U.S. ally in the region – resigned. Following the resignations, control of the country appears to have been turned over to Houthi rebels, a group that is both opposed to U.S. influence and is allegedly supported by Iran.

Yemen is a key area of concern for the United States, in large part due to the presence of the country's much-feared and powerful al-Qaeda affiliate. Yet the Houthis are little known outside the country. What does their success mean?

Who are the Houthis, and where did they come from?

The Houthis are a Shiite insurgency group that originated from northwestern Yemen's Saada province. Charles Schmitz, a professor at Towson University, writes that their origins lie in the Shabab al-Mumanin (the Believing Youth), a group that operated in the early 1990s. The Believing Youth worked to raise awareness about the Zaydi branch of Shiite Islam, which had dominated Yemen for centuries but was sidelined after a civil war in the 1960s and repressed by the Yemeni government.

After the U.S.-led invasion of Iraq in 2003, Hussein al-Houthi, one of the leaders of the Believing Youth, began staging anti-American protests and became a vocal critic of then-president Ali Abdullah Saleh. After Houthi's followers clashed with the government, Yemeni forces killed him. Following his death, the group was renamed after him. The insurgency continued, led by those related to Hussein al-Houthi – 33-year-old Abdulmalik al-Houthi is the current leader.

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How did they become so powerful?

A long period of armed conflict with the government turned the Houthis from "student activists to seasoned insurgents," Schmitz writes, and the government's harsh tactics in the north found them a broad set of allies. While they reached a cease-fire with the government in 2010, the next year large protests against President Saleh offered them a new opportunity. The group took advantage, consolidating their control in the northwest and taking part in the National Dialogue Conference (NDC) after Saleh stepped down from power.

Hadi became president in 2012 after a U.N.-brokered peace deal, yet he suffered from a variety of differing problems, including a southern separatist movement, the growing threat of al-Qaeda in the Arabian Peninsula, and the continuing loyalty of many military officers to Saleh. Again, the Houthis, emboldened by previous successes, pushed on. In September, the group began a dramatic new offensive, eventually gaining a large amount of ground from government forces, and reaching Sanaa, the Yemeni capital.

Intense fighting broke out in Sanaa this week. While it initially looked like a coup, Houthi leaders offered Hadi a power-sharing accord that would have allowed him to stay in power. However, Yemen's leaders balked at the deal and resigned en masse. In a public address, Hadi apologized, saying that “we have reached a dead end.”

Shiite insurgents of the Houthi rebel faction stormed Yemen's presidential palace in Sanaa on Tuesday—a major setback for President Abed Rabbo Mansour Hadi and a blow to the Western-allied government. (The Washington Post)

Is this a sectarian conflict?

Yes and no. The Houthis are Shiite, and many of the forces they are fighting are Sunni. Part of their appeal comes from the idea they are representing Yemen's Shiite minority, which is estimated to account for 35 percent of the country's Muslims.

The reality, however, is more complicated.

For one thing, Zaydi Shiites (almost entirely found in Yemen) are quite distinct from the rest of Shiite Islam, recognizing only the first five of the 12 imams (making them "fivers," as opposed to the "twelvers" of mainstream Shiite Islam). Zaydis are considered to be theologically closer to Sunni Muslims than other Shiites. (It's worth noting that former president Saleh, who ruled Yemen for 12 years and was attacked by the Houthis, is himself a Zaydi and is widely suspected of working with the Houthis after losing power).

Analysts say that the popular appeal of the Houthi insurgency can't entirely be put down to sectarian factors. In a 2010 Rand Corp. report, the authors noted that "it is a conflict in which local material discontent and Zaydi identity claims have intersected with the state center’s methods of rule and self-legitimation." That analysis was echoed last year by Silvana Toska, a Middle East researcher, who noted that the Houthis were supported by "vast numbers of Yemenis who view them as a real opposition to the elites that is untainted by corruption."

What role is Iran playing?

Saudi Arabia and other Sunni states have accused the Houthis of being a proxy for Iran, the region's Shiite superpower. The Houthis themselves deny this. Some outside sources have also suggested a link, however. Last year, Reuters spoke to officials from Saudi Arabia, Yemen and Iran who all claimed that Tehran had supplied money, weapons and training to the Houthis.

One Iranian official told Reuters that there were a "few hundred" members of Iran's Quds Force, the external arm of the Revolutionary Guard, training Houthi fighters in Yemen and that Houthi fighters had traveled to Iran to train as well. The Quds Force is believed to have operated with a number of other Shiite groups, most notably in Lebanon and Iraq.

It's tough to know to what extent Iran actively helps the Houthis, if they actually do. Many have long suspected the Yemeni government and the Saudis of exaggerating the link in a bid to get more U.S. assistance. Notably, as the Yemeni government began to fall, Saudi Arabia, which had supported Sunni groups opposed to the Houthis, cut their aid to the country.

What does it mean for the United States?

It's hard to say. The United States' major concern in Yemen is likely still AQAP, a Sunni extremist group. The Houthis are certainly no friend of al-Qaeda (they have fought the group at points), and Yemen's drone strikes are operated from Saudi Arabia anyway. Even if the Houthis remain dead set against U.S. involvement in the country, if they can form a popular, functioning government their takeover may not be a bad thing.

The problem, of course, is what happens if they can't. As aid from Saudi Arabia dries up, the country's economy will struggle. Without clear authority in Sanaa, groups such as AQAP and the southern secessionist movement have more space to act. And if the Houthis push a Shiite sectarian line, that may push Yemen's Sunnis to follow extremists. What was once a key U.S. ally now seriously risks becoming a failed state.