Timeline: After two years of war, Yemen in crisis
The conflict underscores the regional rivalry between Iran, a supporter of Yemen's Houthi rebels, and Saudi Arabia, which heads the nine-member coalition.
On March 26, 2015, the coalition launched operation "Decisive Storm" with airstrikes against Houthi rebels to defend embattled President Abedrabbo Mansour Hadi, who sought shelter in Riyadh.
The Sunni coalition also comprises the Gulf monarchies Bahrain, Kuwait, Qatar and the United Arab Emirates along with Egypt, Jordan, Morocco and Sudan.
A month later, it is rebranded the operation "Restoring Hope".
The goal was to defeat the rebels who have controlled the capital since September 2014 in addition to large swathes of land in northern, central and western Yemen.
On July 17, 2015, the government announced the liberation of southern Aden province after more than four months of fighting.
In August 2015, the coalition supplemented its air power with hundreds of ground troops.
By mid-August, loyalist forces retook the south, but faced a growing presence of fighters from al-Qaeda and the Islamic State group.
In February 2016, Riyadh said loyalist forces controlled "more than three quarters" of Yemen, despite trouble advancing in the southwestern province of Taiz and in Marib, central Yemen.
Three months of UN-brokered peace talks in Kuwait end in stalemate, and coalition aircraft resume strikes on Sanaa on August 9, 2016.
On January 7, 2017, pro-government troops backed by coalition planes and ships launch operation "Golden Spear" around the strategic Bab al-Mandab Strait, between the Red Sea and the Gulf of Aden.
The UN and US organise three rounds of fruitless peace talks, in June and December 2015 in Switzerland and in April 2016 in Kuwait.
Seven truces are agreed, but all broken.
The severing in January, 2016 of diplomatic relations between Saudi Arabia and Iran complicates the task for diplomats.
|Read also: Why do Yemen's ceasefires keep failing?|
The internationally-recognised government, led by Hadi, establishes a "provisional" base in the port city of Aden in late September 2016.
Two months later, the rebels and allied forces of former president Ali Abdullah Saleh form a government of their own in Sanaa, dousing hopes of a UN-brokered national unity government.
On September 28, 2015, an airstrike smashes a wedding hall in southwestern Mokha, killing 131 people. The coalition denies responsibility.
On August 15, 2016, coalition planes bomb a hospital in Abs, northwestern Yemen, the fourth strike in a year on a medical facility run by the non-governmental organisation Medecins Sans Frontieres (Doctors Without Borders). MSF says 19 people died and 24 were wounded.
Coalition planes have dropped banned cluster munitions and killed two times more civilians than other forces, according to the UN.
On October 8, 2016, an airstrike kills 140 people and wounds 525 others at a funeral in Sanaa. The coalition belatedly acknowledges responsibility.
|Read also: Britain's dark shadow looms over Yemen funeral tragedy|
Washington steps up airstrikes against al-Qaeda in the Arabian Peninsula (AQAP), which has benefitted from the chaos to gain influence.
AQAP loses territory to the Islamic State which on March 20, 2015 claims responsibility for the first time for attacks against two Shia mosques in Sanaa that kill 142 people.
A botched January 29 anti-AQAP raid by US special forces results in the deaths of a US Navy SEAL and multiple civilians – including women and children.
According to the UN, the fighting has displaced more than three million people, and more than two thirds of Yemen's population of around 18.8 million people need aid.
Some 7.3 million people are estimated to be close to starvation and 462,000 children suffer from serious malnutrition. Without $2.1 billion in international aid, the UN warns that Yemen will suffer a famine in 2017.