Yemen, one of the Arab world's poorest countries, has been devastated by a civil war. Here we explain what is fuelling the fighting, and who is involved.
How did the war start?
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.
As president, Mr Hadi struggled to deal with a variety of problems, including attacks by jihadists, a separatist movement in the south, the continuing loyalty of security personnel to Saleh, as well as corruption, unemployment and food insecurity.
The Houthi movement (known formally as Ansar Allah), which champions Yemen's Zaidi Shia Muslim minority and fought a series of rebellions against 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 gradually took over the capital Sanaa.
The Houthis and security forces loyal to Saleh - who was thought to have backed his erstwhile enemies in a bid to regain power - 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 defeating the Houthis, ending Iranian influence in Yemen and restoring Mr Hadi's government.
The coalition received logistical and intelligence support from the US, UK and France.
What's happened since then?
At the start of the war Saudi officials forecast that it would last only a few weeks. But four years of military stalemate have followed.
Coalition ground troops landed in the southern port city of Aden in August 2015 and helped drive the Houthis and their allies out of much of the south over the next few months.
Mr Hadi's government has established a temporary home in Aden, but it struggles to provide basic services and security and the president continues to be based in Saudi Arabia.
The Houthis meanwhile have not been dislodged from Sanaa and north-western Yemen. They have been able to maintain a siege of the third city of Taiz and to launch regular ballistic missile and drone attacks on Saudi Arabia.
In September 2019, Saudi Arabia's eastern oil fields of Abqaiq and Khurais were attacked by air, disrupting nearly half the kingdom's oil production - representing around 5% of global oil output.
The Houthis claimed responsibility but Saudi Arabia and the US accused Iran of carrying out the attacks.
Militants from al-Qaeda in the Arabian Peninsula (AQAP) and the local affiliate of the rival Islamic State group (IS) have taken advantage of the chaos by seizing territory in the south and carrying out deadly attacks, notably in Aden.
The launch of a ballistic missile towards Riyadh in November 2017 prompted the Saudi-led coalition to tighten its blockade of Yemen. It said it wanted to halt the smuggling of weapons to the rebels by Iran - an accusation Tehran denied - but the restrictions led to substantial increases in the prices of food and fuel, helping to push more people into food insecurity.
The alliance between the Houthis and Ali Abdullah Saleh also collapsed in November 2017 following deadly clashes over control of Sanaa's biggest mosque. Houthi fighters launched an operation to take full control of the capital and Saleh was killed.
In June 2018, the coalition attempted to break the deadlock on the battlefield by launching a major offensive to capture from the Houthis the Red Sea city of Hudaydah, whose port is the principal lifeline for almost two thirds of Yemen's population.
The UN warned that the port's destruction would constitute a "tipping point" beyond which it was going to be impossible to avert massive loss of life due to famine.
After six months of fighting, the warring parties agreed a ceasefire at talks in Sweden. The Stockholm agreement required them to redeploy their forces from Hudaydah, establish a prisoner exchange mechanism, and to address the situation in Taiz.
While hundreds of prisoners have since been released, the full redeployment of forces from Hudaydah has not yet taken place, raising fears that the Stockholm agreement will collapse and that the battle for Hudaydah will resume.
In July 2019, the United Arab Emirates (UAE), a key ally of Saudi Arabia in the war, facing international criticism of its conduct, announced a withdrawal of its forces from Yemen.
In August, fighting erupted in the south between Saudi-backed government forces and an ostensibly allied southern separatist movement supported by the UAE, the Southern Transitional Council (STC).
Forces loyal to the STC, which accused Mr Hadi of mismanagement and links to Islamists, seized control of Aden and refused to allow the cabinet to return until Saudi Arabia brokered a power-sharing deal that November.
The UN hoped the agreement would clear the way for a political settlement to end the civil war, but in January 2020 there was a sudden escalation in hostilities between the Houthis and coalition-led forces, with fighting on several front lines, missile strikes and air raids.
In April 2020 the STC declared self-rule in Aden, breaking a peace deal signed with the internationally recognised government, saying it would govern the port city and southern provinces.
Saudi Arabia announced a unilateral ceasefire the same month due to coronavirus pandemic but the Houthis rejected it, demanding the lifting of air and sea blockades in Sanaa and Hudaydah.
What's been the human cost?
In short, Yemen is experiencing the world's worst humanitarian crisis.
The UN had verified the deaths of at least 7,700 civilians by March 2020, with most caused by Saudi-led coalition air strikes.
Monitoring groups believe the death toll is far higher. The US-based Armed Conflict Location and Event Data Project (ACLED) said in October 2019 that it had recorded more than 100,000 fatalities, including 12,000 civilians killed in direct attacks.
More than 23,000 fatalities were reported in 2019, making it the second most lethal year of the war so far.
Thousands more civilians have died from preventable causes, including malnutrition, disease and poor health.
The charity Save the Children estimated that 85,000 children with severe acute malnutrition might have died between April 2015 and October 2018.
About 80% of the population - 24 million people - need humanitarian assistance and protection.
Some 20 million people need help securing food, according to the UN. Almost 10 million of them are considered "one step away from famine".
An estimated 2 million children are acutely malnourished, including almost 360,000 children under five years old who are struggling to survive.
With only half of the country's 3,500 medical facilities fully functioning, almost 20 million people lack access to adequate healthcare. And almost 18 million do not have enough clean water or access to adequate sanitation.
Consequently, medics have struggled to deal with the largest cholera outbreak ever recorded, which has resulted in more than 2.2 million suspected cases and 3,895 related deaths since October 2016.
The United Nations has warned that the death toll from the coronavirus pandemic could "exceed the combined toll of war, disease, and hunger over the last five years."
The UN also issued a desperate plea for financial aid saying its operations in the country, including vital health services, were severely underfunded.
The war has displaced more than 3.65 million from their homes.
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 - such as from al-Qaeda or IS affiliates - emanating from the country as it becomes more unstable.
The conflict is also seen as part of a regional power struggle between Shia-ruled Iran and Sunni-ruled Saudi Arabia.
Gulf Arab states - backers of President Hadi - have accused Iran of bolstering the Houthis financially and militarily, though Iran has denied this.
Yemen is also strategically important because it sits on a strait linking the Red Sea with the Gulf of Aden, through which much of the world's oil shipments pass.