The chaos in Yemen has, in the last few days, gone from bad to much, much worse. The president, after a brief disappearance, appears to be in Saudi Arabia. Rebels control the capital city. And Yemen's neighbors have already started bombing the country and could soon invade.

Here's a guide to the basics of what's going on.

This all started with a rebel uprising in Yemen's north

The Houthi rebel group is the key player in this drama. This Yemeni group has been around since the 1990s and has been in an on-and-off conflict with the central Yemeni government since 2004.

The Houthis are from the country's northwest and are religiously Zaydi, an offshoot of mainstream Shia Islam. Yemen's Zaydis are concentrated in the north, while southern Yemen is largely Sunni. This conflict isn't over religion, but that religious divide ends up being important for understanding what's going on.

Yemen's Zaydi community believes the central government has repressed them and hasn't addressed their interests. "Regionalism — not sectarianism — is still the most decisive factor in Yemen until now," Farea al-Muslimi, a visiting scholar at the Carnegie Endowment for International Peace, writes. "The Houthis revived a sectarian conflict that had nearly gone extinct in Yemen."

That conflict got worse after the 2011 Arab Spring, in which Yemenis toppled dictator Ali Abdullah Saleh. The Houthis supported that uprising. And the chaotic collapse of Yemen's central government helped strengthen their military forces relative to the government.

After Saleh was kicked out, the international community helped set up a transitional government intended to help Yemen establish a stable, permanent new government. Appointed to lead it was Abd Rabbu Mansour Hadi, the transitional president.

But the Houthis "had no representation in the transitional government," Towson University's Charles Schmitz explains. So they saw "the transitional government as no different from the old regime that conducted wars against them — in other words, a body that cannot be trusted."

So the Houthis kept fighting — and still are today. Only now, they're winning.

The rebels have grown so powerful they captured the capital and declared themselves in charge

Throughout most of 2013 and part of early 2014, the Yemeni held a series of meetings to try to find a national consensus on the country's future. But they barely represented the Houthis, though the Houthis were quite powerful.

Outraged, and believing the transitional government was deliberately refusing to represent them, in mid- to late 2014 the Houthis organized demonstrations against the Cabinet and some of the government's specific economic policies. Houthi forces mobilized, and the protests eventually became a full-on military conflict.

Houthi fighters marched on the capital city of Sanaa, where fighting spread by September 18. The Houthis swept away government resistance and established control of much of Sanaa. After occasional clashes in Sanaa, Houthi forces took over the presidential palace in January. The next month, they formally deposed Hadi.

The Houthis have been able to achieve so many victories against Yemen's military in large part because the transitional government is very weak. Yemen is already a poor country, and decades of Saleh's corrupt dictatorship had weakened government institutions.

"No unified and autonomous military, replete with a functioning chain of command and esprit de corps, existed" under Saleh, according to UCLA historian James Gelvin. Saleh's regime's demise only weakened the official forces, creating a power vacuum that the Houthis stepped into.

Now the rebels are advancing south, threatening to consolidate their rule over Yemen

The remnants of Hadi's government are based in Aden, a coastal city in the country's far south. In recent weeks, Houthi forces have pushed in that direction, seizing Yemen's third largest city, Taiz. They're still fighting against forces loyal to Hadi, and the risk that they could push the transitional government out of Yemen entirely is what makes this crisis seem so immediate.

It's not clear whether the Houthis actually have the military might to conquer the whole country. But even if they don't, the possibility of a protracted civil war in Yemen is looking increasingly likely, and scary.

Complicating matters further, the old dictator Saleh, who supposedly stepped down a few years ago, is back — and now he's working with the Houthis.

"[Saleh] has helped lead units of the Yemeni military and security services to swing to the side of the Houthis," the New York Times's David Kirkpatrick reports. "Some of the Houthi allies have even begun calling for the election of the former president’s son, Ahmed Ali Abdullah Saleh, as Yemen’s next leader."

So this isn't a matter of "old regime versus rebels," as you might think. It's elements of the old regime AND sectarian rebels ganging up on forces loyal to what's supposed to be an internationally backed transitional government. So things are not great.

Saudi Arabia and other Middle Eastern countries are turning this into a regional war

On Wednesday evening, a coalition including Saudi Arabia, Egypt, and several Gulf states announced the beginning of an anti-Houthi campaign, turning this into a much larger conflict.

Saudi Arabia initially took the lead in the offensive, as its jets pounded Houthi positions. Now the coalition is threatening a ground invasion of Yemen, using at least Saudi and Egyptian troops, to push back the Houthis. Why are Saudi Arabia and its allies in the Gulf states getting involved in what seems like a mostly local conflict? One word: Iran.

Iran, a Shia theocracy and a fierce rival of Saudi Arabia, has been supporting the Houthis, who are Zaydi Shia. While the Houthis sometimes deny getting Iranian support, it's widely understood that they have ties to the Lebanese militant group Hezbollah, an Iranian proxy. Both Yemeni and Western officials have said Iran's support for the Houthis goes even deeper.

"It's been happening for over a year. We've seen Houthis going out to Iran and Lebanon for military training," an unnamed Western official told Reuters. "We think there is cash, some of which is channeled via Hezbollah and sacks of cash arriving at the airport. The numbers of those going for training are enough for us to worry about."

For the Houthis, this is partly ideological.

"A key aspect of the Houthis' ideology was shoring up Zaydism against the perceived threat of the influence of Saudi-influenced ideologies and a general condemnation of the Yemeni government’s alliance with the United States," Yemen expert Adam Baron writes in Politico Magazine.

The sectarian and political links between Iran and the Houthis have really scared Sunni Saudi Arabia. Houthi rhetoric is deeply anti-Saudi and anti-Western; Yemen, of course, shares a long border with Saudi Arabia. The Saudis and other nearby Gulf states see the Houthi takeover as creating an Iranian bulwark unacceptably close to home.

"The Saudis have always seen themselves as the exclusive outside power in Yemen and to that extent they worry about the Houthis because they see the Houthis as an extension of Iranian influence," Texas A&M's F. Gregory Gause told Susris.

The Saudi decision to support Hadi, then, is really about its fear that the Houthis are an Iranian proxy. This proxy war dynamic is very dangerous. In Syria, Iranian support for Bashar al-Assad's regime and Gulf state support for anti-government and Islamist rebels deepened the conflict and further divided the country on sectarian lines. It also helped give rise to ISIS.

Oh, and one last thing: al-Qaeda's strongest branch, al-Qaeda in the Arabian Peninsula (AQAP), is based in southern Yemen. The Hadi government had been a strong American counterterrorism partner, cooperating with American bombing campaigns against AQAP. The Houthis also despise AQAP, but they're more focused on fighting the Yemeni government, and that fight has forced the US to withdraw some of its counterterrorism efforts there, thus giving al-Qaeda a freer hand.

Currently, the US is backing the Gulf state offensive in support of Hadi with, according to the White House, "logistical and intelligence support." The supposedly local Yemeni conflict, in other words, just got a whole lot more international.

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