Yemen crisis: Who is fighting whom?
Yemen, one of the Arab world's poorest countries, has been devastated by a war between forces loyal to the internationally-recognised government of President Abdrabbuh Mansour Hadi and those allied to the Houthi rebel movement.
More than 7,600 people have been killed and 42,000 injured since March 2015, the majority in air strikes by a Saudi-led multinational coalition that backs the president.
The conflict and a blockade imposed by the coalition have also triggered a humanitarian disaster, leaving 70% of the population in need of aid.
How did the war start?
The conflict has its roots in the failure of the political transition that was supposed to bring stability to Yemen following an uprising that forced its longtime authoritarian president, Ali Abdullah Saleh, to hand over power to Mr Hadi, his deputy, in November 2011.
Mr Hadi struggled to deal with a variety of problems, including attacks by al-Qaeda, a separatist movement in the south, the continuing loyalty of many military officers to Mr Saleh, as well as corruption, unemployment and food insecurity.
The Houthi movement, which champions Yemen's Zaidi Shia Muslim minority and fought a series of rebellions against Mr Saleh during the previous decade, took advantage of the new president's weakness by taking control of their northern heartland of Saada province and neighbouring areas.
Disillusioned with the transition, many ordinary Yemenis - including Sunnis - supported the Houthis and in September 2014 they entered the capital, Sanaa, setting up street camps and roadblocks.
In January 2015, the Houthis reinforced their takeover of Sanaa, surrounding the presidential palace and other key points and effectively placing Mr Hadi and his cabinet ministers under house arrest.
The president escaped to the southern port city of Aden the following month.
The Houthis and security forces loyal to Mr Saleh then attempted to take control of the entire country, forcing Mr Hadi to flee abroad in March 2015.
Alarmed by the rise of a group they believed to be backed militarily by regional Shia power Iran, Saudi Arabia and eight other mostly Sunni Arab states began an air campaign aimed at restoring Mr Hadi's government.
The coalition received logistical and intelligence support from the US, UK and France.
What's happened since then?
After two years of fighting, no side appears close to a decisive military victory.
Pro-government forces - made up of soldiers loyal to President Hadi and predominantly Sunni southern tribesmen and separatists - were successful in stopping the rebels taking Aden, but only after a fierce, four-month battle that left hundreds dead.
Having established a beachhead, coalition ground troops landed in Aden that August and helped drive the Houthis and their allies out of much of the south over the next two months. Mr Hadi and his government returned from exile at the same time and established a temporary home in Aden.
But since then, despite the air campaign and naval blockade continuing unabated, pro-government forces have been unable to dislodge the rebels from their northern strongholds, including Sanaa and its surrounding province.
The Houthis have also been able to maintain a siege of the southern city of Taiz and to continue firing missiles and mortars across the border with Saudi Arabia.
Jihadist militants from al-Qaeda in the Arabian Peninsula (AQAP) and rival affiliates of so-called Islamic State (IS) have meanwhile taken advantage of the chaos by seizing territory in the south and stepping up their attacks, notably in government-controlled Aden.
What's been the impact on civilians?
Civilians have borne the brunt of the fighting and repeatedly been the victims of what activists have described as serious violations of international law by all parties.
By 26 March 2017, at least 4,773 civilians had been killed and 8,272 others injured, according to the United Nations. With just under half of the population under the age of 18, children constituted a third of all civilian deaths during the first two years of the conflict.
The destruction of civilian infrastructure and restrictions on food and fuel imports have also pushed Yemen to the brink of famine.
Some 17 million people are considered food insecure and 6.8 million severely food insecure.
About 3.3 million children and pregnant or breast-feeding women are acutely malnourished, including 462,000 children under five who face severe acute malnutrition.
Only 45% of the 3,500 health facilities surveyed by the UN in November were fully functioning.
Why have peace efforts failed?
The UN has organised three rounds of peace talks. There was hope of a breakthrough at the last round, which opened in Kuwait in April 2016, with both the Houthis and the Saudis seemingly under pressure and willing to negotiate.
However, the talks collapsed three months later, triggering an escalation in the fighting that the UN said resulted in the number of civilian casualties rising dramatically.
Mr Hadi's government says the political process can only proceed if UN Security Council resolution 2216, which calls for the rebels to withdraw from all areas they control and lay down their arms, is fully implemented.
Why should this matter for the rest of the world?
What happens in Yemen can greatly exacerbate regional tensions. It also worries the West because of the threat of attacks emanating from the country as it becomes more unstable.
Western intelligence agencies consider AQAP the most dangerous branch of al-Qaeda because of its technical expertise and global reach, and the emergence of IS affiliates in Yemen is a serious concern.
The conflict between the Houthis and the government is also seen as part of a regional power struggle between Shia-ruled Iran and Sunni-ruled Saudi Arabia.
Gulf Arab states have accused Iran of backing the Houthis financially and militarily, though Iran has denied this, and they are themselves backers of President Hadi.
Yemen is strategically important because it sits on the Bab al-Mandab strait, a narrow waterway linking the Red Sea with the Gulf of Aden, through which much of the world's oil shipments pass.