Yemen's crisis cuts through the country's political, tribal, regional and sectarian layers to create a complex conflict that risks sucking in neighbouring oil giant Saudi Arabia and its main regional rival, Iran.
These are some of the most important factions in Yemen's crisis.
The Houthis, or Ansarullah. The group began as a movement of young men called the Believing Youth set up in 1992 to back the rights of the Zaydi Shi'ite sect that makes up around a fifth of Yemenis and it fought the government from 2003-09. It recently claimed the mantle of a national revolution and swept southwards, seizing Sanaa. The Houthis are allied to Iran, but the extent of the relationship is a matter of speculation.
Abd-Rabbu Mansour Hadi. Elected as an interim president in 2012 to lead a political transition towards democracy, Hadi's residence was besieged by the Houthis after they seized Sanaa. He resigned in January and was put under house arrest, but escaped last month to Aden, repudiated his resignation and formed a government there, and called on the army to join him.
Ali Abdullah Saleh. The ruler of north Yemen from 1978 and of the unified state from 1990 was forced to concede power in 2011 after mass protests, although he stayed ceremonial president until 2012. Western countries accuse him of using his wide influence, military power base, and an unlikely alliance with the Houthis to undermine Hadi in a bid to win back power.
Al Qaeda in the Arabian Peninsula. AQAP has been the most active wing of the Islamist militant movement for years, plotting attacks on international airliners and launching raids into Saudi Arabia. Despite repeated army campaigns to oust it from its strongholds in the south and east, it has carried out a string of deadly attacks against Yemen's security forces.
Southern Hirak. The movement is an unwieldy coalition of groups who want to reverse the state's 1990 reunification and revive the old South Yemen. Hirak can mobilise large numbers of people in the streets of southern cities such as Aden, but has no coherent leadership to translate its popular support into action.
Islah. A party which combines Islamist and tribal interests, Islah has widespread support across Yemen and looked poised to win more power during the transition, but it lost out in the Houthi advance. Its military might came from an alliance with General Ali Mohsen al-Ahmar, who held the loyalty of key brigades, but has fled to Saudi Arabia.