The Houthis, rebels based in the northern region of Yemen, have been trying to take control of the country since 2004, well before the Arab Spring. The Shiite insurgent group, led by Abdulmalik Houthi, is by far the strongest rebel force in the country.
Since September, the group has launched an increasing number of attacks in protest against what it believes is discrimination from the government of Yemen, which is almost two-thirds Sunni. Apart from their geographical control, the rebels have also seized "state-run media outlets and government buildings, notably the headquarters of the Yemeni intelligence service."
On Tuesday, the group, allegedly backed by Iran, launched fierce attacks in the capital, Sanaa, and was able to overrun Yemen's presidential palace.
The Arab Spring
After the Arab Spring bloomed across the Middle East in 2011 in the wake of Tunisians' overthrow of their dictator, pro-reform demonstrations erupted in Yemen. President Ali Abdullah Saleh, who had been the leader of the country for more than 20 years, refused to step down. A month after the U.N. Security Council passed a resolution in October condemning the violence in the country and calling for a transfer of power, Saleh handed over power to his deputy, Abed Rabbo Mansour Hadi.
Hadi, who has now reportedly resigned, was a strong ally of the United States. He struggled with his military after he tried to get rid of officers who appeared to be loyal to his predecessor, Saleh. The Houthis have become more powerful in part because of the faltering military.
Al-Qaeda in the Arabian Peninsula
Seen as "al-Qaeda's most capable wing," this Yemen-based terror group, known as AQAP, has been in the news lately because of claims that it was responsible for the attacks in Paris against the Charlie Hebdo satirical newspaper that left 12 dead. AQAP has been the target of drone strikes by the Obama administration and has been able to carry out plots that directly threaten the United States and its allies.
Hadi's administration has been working with the United States to fight AQAP. Al-Qaeda and its Sunni allies are therefore at odds with Hadi's government, as well as with the Houthis, who are Shiites.
The Sunni-Shiite divide
To summarize: The Houthis, a Shiite rebel group, represent a serious threat to the U.S.-backed Sunni government. This poses a huge problem for the Obama administration, which has been actively fighting the al-Qaeda faction in Yemen. This faction is also at odds with the Houthis, so if this rebel group does take control, it could mean a much-diminished role for the United States and cause Yemen to plunge into a dangerous civil war.