BEIRUT (Reuters) - Each of the parties to Yemen's messy civil war has its own agenda, making the conflict harder to resolve. This factbox looks at some of these groups and what they want.
In the late 1990s, the Houthi family 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.
As friction with the government grew, they fought a series of guerrilla wars with the national army and a brief border conflict with Saudi Arabia. They built ties with Iran, but it is not clear how deep that relationship goes.
Since seizing the capital, Sanaa, in 2014, the Houthis have relied on parts of the existing bureaucracy to govern, and their long-term strategy is unclear.
LOYALISTS OF THE LATE PRESIDENT SALEH
Ali Abdullah Saleh took power in north Yemen in 1978 and after unification with the south in 1990 he stayed on as president. He joined with tribal power brokers to dominate the country, placing his clansmen in key positions in the army and economy, prompting accusations of massive corruption.
When former allies deserted him during the Arab spring, forcing him from power, Saleh disrupted the political transition and joined with his erstwhile foes, the Houthis, helping them seize Sanaa.
Despite their differences, they ruled much of Yemen together until last year. Then Saleh saw a chance to regain power for his family by turning on the Houthis but was killed trying to flee.
When Saleh switched sides, so did some commanders and troops loyal to him. They are now fighting against their former Houthi allies under the late president's son Ahmed, an army general with ties to the UAE.
PRESIDENT HADI'S GOVERNMENT
A general in south Yemen before unification, Hadi sided with Saleh during the brief 1994 civil war. After defeating the separatists, Saleh made Hadi vice president.
When Saleh was forced from power, Hadi was elected to a two-year term in 2012 to oversee a transition to democracy with a new constitution and new elections scheduled for 2014.
But his government was constantly undermined by Saleh, faced a renewed southern separatist movement, Houthi rebellion and many bombing attacks by al Qaeda. The Houthis rejected the new constitution and elections were shelved.
After the Houthis took Sanaa, Hadi fled and then went into exile in Saudi Arabia. He lacks any power base of his own and is widely seen as weak. But the Arab coalition's military intervention is based on upholding the legitimacy of his internationally recognised government and the political transition vested in it.
After independence from Britain, South Yemen became the only Communist country in the Middle East, but it suffered constant infighting. Weakened by that and the collapse of the Soviet Union, it unified with Saleh's North Yemen in 1990.
As it became clear most power was in northern hands, the old southern leadership tried to secede in 1994, but was swiftly beaten by Saleh's army, which sacked Aden. Many southerners have complained of increasing economic and political marginalisation.
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 fighters against the Houthis.
AL QAEDA IN THE ARABIAN PENINSULA
Set up by members of the global jihadist group who had escaped prison in Yemen and their comrades who fled Saudi Arabia last decade, al Qaeda in the Arabian Peninsula became one of its most powerful branches.
It took advantage of Arab Spring chaos to create mini states in remote eastern regions and launched numerous bloody attacks that undermined Hadi's transitional government.
During the civil war, it has carried out attacks against both sides. Any prolonged chaos in Yemen will give it more room to consolidate and plot attacks abroad.
SAUDI ARABIA, THE UAE AND THE ARAB COALITION
Saudi Arabia regards the Houthis as a proxy for Iran, its greatest regional rival, and it wants to stop Tehran gaining sway in its neighbour.
Its other war aims are to end Houthi missile strikes and shelling, to re-start the transition process it backed in 2012, and to prevent long-term chaos in Yemen that would let AQAP plot attacks on the kingdom.
Saudi Arabia's air force has been particularly involved in Yemen, but the kingdom has also provided a base-in-exile for Hadi and assisted the ground fighting in northern Yemen.
The UAE, which also backed the 2012 transition plan, is the other main participant in the coalition. It wants to prevent Islamist militancy growing in Yemen and regards the country's ports as strategically important. It has focused on the war in southern Yemen and the Red Sea coast.
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 Tehran'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 Tehran has denied funnelling weapons into Yemen.
(Reporting By Angus McDowall, editing by Larry King)
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Updated Date: Jun 17, 2018 00:05 AM