Yemen war explained
Ansar Allah, - Houthis
Ansar al-Sharia, Al-Qaeda in the Arabian Peninsula
Saudi coalition, ex-President Abdrabbuh Mansur Hadi
The conflict has its roots in the failure of a political transition supposed to bring stability to Yemen following an Arab Spring uprising that forced its longtime authoritarian president, Ali Abdullah Saleh, to hand over power to his deputy Abdrabbuh Mansour Hadi, in 2011.
President 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 late 2014 and early 2015, the rebels took over Sanaa.
The president escaped to the southern port city of Aden the following month.
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.
In short, the situation in Yemen is, the UN says, the world's worst man-made humanitarian disaster.
Combined with the Saudi-led coalition blockade, the targeting of Yemen’s food production has worsened the world’s worst humanitarian crisis. The deliberate targeting of food production and distribution in Yemen confirms that the coalition is not even trying to avoid hitting civilian targets in many instances. The repeated targeting of medical facilities in Yemen is also no accident. Coalition bombing of water and sewage treatment plants has contributed to the spread of preventable diseases and helped to create the worst cholera epidemic of modern times
More than 9,245 people have been killed and 52,800 injured since March 2015, the UN says.
At least 5,558 of those killed, and 9,065 of those injured up to 14 December 2017 were civilians. Saudi-led coalition air strikes were the leading cause of overall civilian casualties.
According to the UN Human Rights Council, civilians have repeatedly been the victims of "unrelenting violations of international humanitarian law".
Saudi-led coalition bombing has hit civilian targets more than 30% of the time, and that may be a conservative estimate. The coalition has bombed many schools, houses, weddings, funerals, medical facilities, and factories since 2015, and their campaign has targeted critical infrastructure needed to transport food and medicine to the most populated areas of the country. The Saudis and their allies are credibly accused of committing numerous war crimes in Yemen with their indiscriminate bombing campaign
About 75% of the population - 22.2 million people - are in need of humanitarian assistance, including 11.3 million people in acute need who urgently require immediate assistance to survive - an increase of 1 million since June 2017.
Some 17.8 million people do not know where their next meal is coming from and 8.4 million are considered at risk of starvation. Severe acute malnutrition is threatening the lives of almost 400,000 children under the age of five.
With only half of the country's 3,500 health facilities fully functioning, at least 16.4 million people are lacking basic healthcare.
Medics have struggled to cope with the world's largest cholera outbreak, which has resulted in more than 1 million suspected cases and 2,248 associated deaths since April 2017.
More than 3 million people have been forced to flee from their homes in the past three years, including 2 million who remain displaced.
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.