In recent months Houthi rebels from the north have swept through Yemen, capturing the capital, forcing the president into exile and laying siege to the southern port of Aden. Are geographical rivalries the key to the conflict?
The Houthi are Shia Muslims, supported by Iran. The rest of Yemen is mostly Sunni Muslim, and Saudi Arabia is leading a bombing campaign against the Houthi forces. So is this a sectarian conflict, or even a regional proxy war?
And the Houthis have allied with former President Saleh against Yemen's current leader who replaced him after Yemen's 2011 revolution. Are the roots of the current conflict in the failure of that revolution?
The BBC World Service's The Inquiry programme hears from four expert witnesses.
Sarah Jamal: Conflict fuelled by disappointment after the revolution
Sarah Jamal lives in Sanaa, the capital of Yemen, and supported the 2011 revolution.
"This is not a sectarian conflict. People right now are not killing each other for the sake of religion.
"I didn't know my sect until 2011. Growing up I prayed side by side with friends from different backgrounds. We don't have separate schools when it comes to religion. We have a unified religious curriculum, we take the same courses.
"The first days of the revolution were 'the honeymoon', where people came together from different backgrounds.
"It was beautiful. The tribes would come to the square from outside the capital. We would search them for weapons and they would gladly lift their arms and chant for peace. We ate from the same plate, we cried over strangers that were shot, we protected each other and we remained peaceful and loyal to those goals.
"[The previous regime] was a system that sucked everything out of the country. There were a lot of resources. All of that went to the one percent, to Saleh's family and his allies. For 33 years of atrocities he was rewarded with immunity.
"The national dialogue ignored any practical steps towards transitional justice on the ground. Houthis meanwhile changed from victims - a group that was stripped of basic rights - into an armed militia. The southerners saw nothing on the ground, their lands were not restored, their jobs were not restored and the southern movement was never heard.
"The current conflict is 100% related to disappointment after the revolution. People took to the streets, they had demands. The demands were not met. There was no justice, there was no reconciliation, there's pure revenge now.
"I was okay with getting killed in 2011 because to me that was a sacrifice for a better future. But getting killed now would just make me collateral damage."
Baraa Shiban: The Houthis want vengeance
Baraa Shiban works for the charity Reprieve in Yemen but is currently stranded in London.
"[The conflict is due to] an accumulation of failed policies of a 33-year-old dictatorship. Former President Saleh decided to run a number of conflicts to legitimise his presence in power.
"If you read the books of [Houthi founder] Hussein al-Houthi, he focuses on the US invasion of Iraq. He explains that the movement should stop US occupation on our lands. This resonates with many followers, who believe that there have been many injustices done by the US and its allies in many Muslim countries.
"Almost 40,000 people died in President Saleh's 2004 war against the Houthis. This created huge grievances amongst the people in those areas who felt that the Yemeni government didn't care about their grievances, didn't care if their lives had been destroyed, and were willing to keep this war going forever.
"I was shocked by the amount of destruction I saw when I went to the Houthi stronghold, Saada. The main city was totally destroyed.
"The Yemeni government didn't have any forces in Saada, leaving it to rule itself. And that's when the Houthi movement stepped in and started to provide security and basic judiciary for the people.
"They started to believe that they could control more cities and provinces, and they could get followers from the other provinces going down to the capital.
"The Houthis see themselves as a new power that can replace the government.
"My fear is it's easy to ignite the sectarian conflict, and frame this as a war based on sectarian grounds."
Summer Nasser: Southern grievances must be addressed
Summer Nasser is a Yemeni-American student who should be in New York but is stuck at her family's house in the south of Yemen.
"It's the street wars that are more terrifying. You have tanks that are shooting randomly in the city itself and you have shooting between the people who are anti-army, anti-Houthi, and there are Houthi fighters attacking them."
The north and south of Yemen were unified as one country in 1990.
"Actually southerners were very happy about [reunification], and they felt it was a step to develop the country on both ends. "
But four years later, dissatisfaction with the new unified government led by President Saleh led to civil war:
"Land, homes were being taken, not a small amount of land. Southern people that were posted in the military ended up being dropped out of the military to give positions to the northerners.
"In 2007 the southern movement was officially created. The southerners did not actually call for separation, they called for attention to the southern case. That didn't happen. They felt fed up and that's when independence calls really pushed forward.
"After it failed, they ended up going back to the squares, they did civil disobedience, constant protests, almost daily. They looked at it as another failure and that's why they ended up pushing more for the separation.
"The current conflict can absolutely not be resolved without addressing the grievances of the south.
"People are saying in the media that it's a sectarian conflict. Yemenis do not believe that. It's not sectarian whatsoever, we have lived together peacefully - Shias and Houthis and Zaidis - for years. It's about power and greed."
April Longley Alley: Regional power struggle
April Longley Alley is senior Arab Peninsula analyst for the International Crisis Group, based in Dubai.
"Saudi Arabia is playing a pivotal role at the moment. Yemen is in Saudi's backyard, so from the Saudi's perspective, when they look at Yemen, they are convinced that the Houthis are an Iranian proxy. So as the Houthis expanded rapidly militarily, this was perceived as a direct threat.
"The Houthis are a Yemeni movement, [but] they have expressed respect for the Iranian regime. They clearly have political ties with the Iranian regime and moral support. There seems to be some degree of financial support, possibly military training, but what we're seeing now appears to be a self-fulfilling prophecy.
"The Houthis have taken extremely inflammatory actions to stoke those fears that there is this very strong connection between the two, and as the Saudis become more involved in Yemen, it's being pulled into the regional conflict between Saudi and Iran, which is only going to complicate and prolong conflict inside of Yemen itself in a situation where nobody will win.
"Unfortunately the conflict is already taking on a sectarian dimension, so the addition of regional players is stoking a sectarian narrative and sectarian language that is mostly foreign to Yemen. The fabric of Yemeni society and those bonds that have held these groups together are being pulled apart.
"At its core this is a political power struggle inside of Yemen. But various actors inside of Yemen have certainly opened their country up to this regional power struggle. This is a conflict inside of Yemen in which no one group is going to come out as a clear military victor.
"There is no military solution to this conflict. It must be a political compromise or we're in for a very long period of violence."