Who Are the Houthis of Yemen?

The Houthis, an insurgent group based in northwestern Yemen, have been fighting the government intermittently since 2004, when the country was still led by President Ali Abdullah Saleh, a longtime autocrat and United States ally.

While Yemen is mostly Sunni, the Houthis belong to an offshoot of Shiite Islam known as Zaydism. After the American-led invasion of Iraq, their leader, Hussein al-Houthi, capitalized on popular anger to begin a revolt against Mr. Saleh. He was killed in 2004, but his followers, in the northern region of Saada, continued the fight until a 2010 cease-fire.

Saudi Arabia also carried out unilateral military operations against the Houthis along its border with Yemen in 2009.

Mr. Saleh was overthrown in 2012 as the Arab Spring roiled the region. He was replaced by his longtime vice president, Abdu Rabbu Mansour Hadi, in a peaceful transfer of power orchestrated by the Gulf countries and the West.

Since then, the Houthis have grown into a broad national political movement that is battling a reinvigorated Al Qaeda in the Arabian Peninsula. Supporters were hopeful that the Houthis could tackle rampant corruption and restore the stability that has eluded Yemen since the Arab Spring.

In September, Houthis seized control of Sana, the capital, promising radical political changes. But they were met with almost daily retaliatory attacks from Al Qaeda, including a bombing of a Houthi rally that killed more than 40 people in October.

Yemeni officials and diplomats say the Houthis, now led by Abdul-Malik al-Houthi, 33, are backed by Iran. The Houthis deny that link, but Yemeni authorities have seized ships carrying Iranian weapons that they allege were being sent to the Houthis.

The government grew weaker in recent months as the Houthis claimed greater control in several ministries. Many observers fear intervention by regional rivals like Saudi Arabia and Iran could cause further unrest. Al Qaeda has used searing anti-Shiite rhetoric to describe the Houthis, leading to additional fears of sectarian bloodletting.

The United States has continued to back Mr. Hadi, while the Houthis have been sharply critical of American drone strikes against Al Qaeda targets in Yemen. The news that at least one of the attackers on the satirical newspaper Charlie Hebdo in Paris had trained in Yemen underscored how dangerous it could be for the West to be without a reliable partner in counterterrorism operations there.

Meanwhile, Yemen continues to be the poorest country in the Middle East, with one of the highest rates of child malnutrition in the world.

“Chronic challenges have become emergencies as the state’s presence in much of Yemen has started to dissolve,” Kareem Fahim wrote in December.