Five years since a Saudi-led coalition of Arab states launched a military intervention against Houthi rebels in Yemen, the movement continues to make advances in the country's north.

In recent weeks the group has taken control of territory in Jawf province including its main city of Hazm, which lies northeast of the capital, Sanaa, while it has also pushed into parts of the resource-rich Marib province, the last stronghold of Yemen's internationally recognised government in the north.

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The Houthi advances against forces loyal to President Abd-Rabbu Mansour Hadi and their regional allies have come despite the financial power of the Saudi-led coalition and the continuing international and domestic isolation of the rebel movement.

Buoyed by his group's military progress in recent months, Houthi leader Abdelmalek al-Houthi has urged the coalition, led by Saudi Arabia and the United Arab Emirates, to halt attacks.

Impact of intervention

Before the coalition started air raids on March 26, 2015, the Houthis had forced President Hadi to flee to the southern city of Aden after holding him temporarily under house arrest.

They had also taken over the majority of Yemen's populous northern and central highlands. Houthi control of the country's air force meant that they were able to bomb pro-government forces in Aden, and almost took full control of the city. Hadi fled to Saudi Arabia the day before Riyadh launched its aerial intervention.

Within months, Saudi Arabia, the UAE and their allies on the ground pushed the Houthis out of southern Yemen towards their northern heartland.

"The most important success of the war has been preventing the Houthis from controlling all of Yemen, or the majority of it, especially the oil and gas-rich areas," said Abdulnaser Almuwadea, a Yemeni political researcher.

"The Houthis have been unable to get international recognition, which would have been a possibility if the Houthis had taken control of Yemen and not faced any local resistance."

But the intervention and the protracted conflict has caused what the UN describes as the world's worst humanitarian crisis in the poorest country in the Middle East.

More than 100,000 people have been killed in the war, according to ACLED (Armed Conflict Location and Event Data), including 12,000 civilians. According to the World Food Programme, 24 million Yemenis are in need of humanitarian assistance, while 20 million are food insecure.

"Air strikes hitting civilian areas and infrastructure have led to fewer Yemenis supporting the war, and the Houthis have been able to take real advantage and grow their base," Almuwadea said.

"The weakness of the Yemeni government and the absence of its real presence on the ground in many areas ... has strengthened the Houthis and made their control of the north more deep-rooted."

Houthi rebel chant slogans as they hold their weapons during a gathering aimed at mobilising more fighters [Hani Mohammed/The Associated Press]

Splits in coalition

The Houthi grip on Sanaa and the northern highlands appears fairly secure but the group has few domestic allies, having fought against most of the other major factions in Yemen, including Hadi loyalists, southern separatists, pro-Islah party militias, and loyalists of the former president Ali Abdullah Saleh.

Saleh helped the Houthis take control of Sanaa in 2014 but was killed in December 2017 by Houthi fighters, who suspected that he was preparing to ally with the Saudi-led coalition.

His death marked the end of a brief period of fighting between the Houthis and forces loyal to the former leader, and perhaps the last real opportunity for the Saudi-led coalition to defeat the Houthis in their northern stronghold.

While Saleh's death led to some coalescence of non-Houthi forces around the coalition Hadi's government, the anti-Houthi alliance was weak and has since fractured.

When the coalition advanced on the Houthi-held port city of Hodeidah in the second half of 2018, it seemed likely to result in a military victory for the alliance, but international fears that the fighting would cause a humanitarian catastrophe allowed diplomatic pressure stopped the advance.

Since then, anti-Houthi forces have turned their guns on each other, as divisions between officially allied groups played out in.

The secessionist Southern Transitional Council (STC), a UAE-backed group that has widespread support across southern Yemen, forced government forces out of its temporary capital of Aden in August 2019.

This led to the worst fighting between anti-Houthi forces since the beginning of the war, as clashes spread across southern Yemen. It also led to increasingly apparent divisions between Saudi Arabia and the UAE, who supported the government and the STC, respectively.

A Saudi-brokered agreement in November 2019 was supposed to bring the two sides back together, but has yet to be implemented fully.

"The anti-Houthi alliance on the ground has deep ideological differences that may not be easy to overcome," said Fatima Abo Alasrar, a non-resident scholar at the Middle East Institute.

"Hadi's government will have to quickly incentivise other factions to unite their efforts under one banner, but this might be too ambitious to expect at this stage of Yemen's conflict."

The STC itself is still adamant that it will not back away from pushing for southern secession.

"The STC is the political representative of the Southern people, and will negotiate to end the crisis in Yemen in a way that ensures a just solution to the Southern issue," Nasr Alesayi, a member of the STC, told Al Jazeera.