BEIRUT • Each of the parties to Yemen's messy civil war has its own agenda, making the conflict harder to resolve.
The Shi'ite Houthis in far north Yemen set up a religious revival movement for the Zaydi sect of Shi'ite Islam, which had once ruled Yemen, but whose northern heartland had been marginalised. They have fought a series of guerrilla wars with the national army and a brief border conflict with Saudi Arabia. They seized Yemen's capital, Sanaa, in 2014. They have built ties with Iran, but it is not clear how deep that relationship goes.
LOYALISTS OF THE LATE PRESIDENT ALI ABDULLAH SALEH
Mr Saleh took power in north Yemen in 1978 and after unification with the south in 1990, he stayed on as president. When former allies deserted him during the Arab spring, forcing him from power, Mr Saleh joined with the Houthis, helping them seize Sanaa. Mr Saleh saw a chance to regain power for his family by turning on the Houthis last year, but was killed trying to flee. Some commanders and troops loyal to him are now fighting against their former Houthi allies under the late president's son Ahmed, an army general with ties to the UAE.
PRESIDENT ABEDRABBO MANSOUR HADI'S GOVERNMENT
A general in South Yemen before unification, Mr Hadi sided with Mr Saleh during the brief 1994 civil war. When Mr Saleh was forced from power, Mr Hadi was elected to a two-year term in 2012 to oversee a transition to democracy. The Houthis rejected the new Constitution and elections were shelved. After the Houthis took Sanaa, Mr Hadi went into exile in Saudi Arabia.
The southern separatist movement has remained internally divided, but it is a powerful force across the old South Yemen and has provided many of the anti-Houthi fighters.
AL-QAEDA IN THE ARABIAN PENINSULA
Set up by members of the terror group who had escaped prison in Yemen and their comrades who fled Saudi Arabia last decade, Al-Qaeda in the Arabian Peninsula has become one of its most powerful branches.
THE ARAB COALITION
Saudi Arabia regards the Houthis as a proxy for Iran and wants to stop Teheran gaining sway in its neighbour. Its other war aims are to end Houthi missile strikes and shelling, and to restart the transition process it backed in 2012. The UAE, which also backed the 2012 transition plan, is the other main participant in the coalition. The other countries in the coalition have been less closely involved, though Sudan has put some troops on the ground.
IRAN AND ITS REGIONAL ALLIES
Iran champions the Houthis as part of its regional "axis of resistance", and the movement has adopted elements of Teheran's revolutionary ideology. But while Saudi Arabia and its allies accuse Iran of arming and training the Houthis, the extent of the relationship is disputed and Teheran has denied funnelling weapons into Yemen.