But more than three-and-a-half years later the Huthi rebels remain undefeated and the conflict is a quagmire that has set off a major humanitarian crisis.

Here is a look at how the outgunned rebels have clung on:

- Who are the Huthis?

The Huthis come from the minority Zaidi Shiite sect of Islam and have their traditional stronghold in the mountainous north of Yemen.

The movement, which takes its name from late spiritual leader Badreddin al-Huthi and his son Hussein, rose up in the 1990s over alleged sectarian discrimination.

Between 2004 and 2010, the Huthis fought six wars against Yemen's then-government and battled Saudi Arabia in 2009-2010 after storming over the border.

Officially calling themselves the Ansarullah (Supporters of God), the Huthis took part in the Arab Spring protests that forced veteran ruler Ali Abdullah Saleh from office in 2012.

In the chaos that followed they later sided with former foe Saleh and together ousted the internationally-recognised government of Abedrabbo Mansour Hadi that had taken power.

The rebel takeover of capital Sanaa set off the Saudi-led intervention in March 2015.

The Huthis later fell out dramatically with Saleh and killed the former president in December 2017.

- Does Iran support the Huthis?

The war in Yemen is viewed by many as a front in the broader struggle between regional titans Saudi Arabia and Iran that has heated up under Crown Prince Mohammed bin Salman in Riyadh.

The Saudis and their ally the United States say Shiite Iran provides military support to the rebels, including components for ballistic missiles that have been fired over the Saudi border.

Tehran denies the accusation and insists its backing for the Huthis is purely political.

Brigadier Jamal al-Moammari, a former Yemeni air force officer, told AFP that Iranian arms, experts and "equipment to develop ballistic missiles" arrived in 2015.

Security analyst Aleksandar Mitreski says Iran helps the Huthis both directly and indirectly.

"Where and when possible, Iran supplies the rebels with equipment and training for the Huthis to remain a formidable opponent," he said.

The Saudi-led coalition has claimed that members of Iran-backed Lebanese militia Hezbollah were also killed in Yemen while training the Huthis. Hezbollah has denied this.

- Arms taken from government?

Despite the allegations over Iranian support, it appears that the bulk of the Huthi's weaponry comes from looted official stockpiles.

Brigadier Abdo Majli, a spokesman for pro-government forces, told AFP that "90 percent of Huthi arms came from the Yemeni army depots" taken along with Sanaa in 2014.

Majli said that despite coalition warplanes destroying some of the weapons the Huthis had managed to "hide them in secret stores" in their northern strongholds.

The rebel's impressive arsenal has been on display in the latest battles for the key port city of Hodeida as they deployed tanks to halt the advance of pro-government forces.

The Huthis have also laid a large number of landmines and manufacture some of their own weapons, including rockets and reportedly even drones.

- Local backing?

The Huthis have benefited from home advantage and local alliances as they have faced off against some of the best equipped militaries in the region.

Despite hailing from northern Yemen, they know the rest of the country and the terrain very well, said analyst Mitreski.

"Aside from geography, what aids the rebels is cooperation with local tribes. Yemen remains a fragmented society along tribal lines, and the rebels capitalise on that," he said.

"Local tribal support goes a long way in this conflict."

The International Crisis Group said in a report in November that the Saudi-led coalition has underestimated the resilience of the Huthis.

"The Huthis are resourceful, committed, experienced and ruthless, and that the core fighters are likely to fight until the last man if called upon to do so," the group said.