Every soldier in the Yemeni city of Aden is on edge. The main checkpoint on the coastal road has two large holes in the roof from mortar shells and the beach has been dug up into berms to slow the advance of any hostile vehicles.

The problem is that the 25 men milling around their posts are not sure what their enemy looks like. In the recent fighting which saw the separatist Southern Transitional Council (STC) eject from the city troops loyal to the exiled Yemeni president – their former allies – al-Qaida took advantage of the chaos, putting on STC uniforms to ambush the soldiers here.

Facebook Twitter Pinterest STC forces at a checkpoint controlling an entrance to the port city of Aden, on 8 September, 2019.

“They raided every checkpoint up to this one,” said Munier al-Atifeh, 42, an STC soldier. “Terrorists, Islah [the Islamist bloc of the Yemeni government], what does it matter? They all want the same thing, which is to keep the south weak.”

More than four years in, Yemen’s conflict is now actually three wars in one: a fight between the Iran-backed northern Houthi rebels and the Saudi-led coalition of Arab nations; a fight between both those parties and extremist elements such as al-Qaida and Islamic State; and belatedly, a fight between the STC, which wants a return to independence for south Yemen, and its former Yemeni government allies.

Facebook Twitter Pinterest The ruins of a mosque that was bombed in 2015 during clashes with Houthi forces for control of the port city of Aden, seen on 9 September, 2019.

The Guardian was invited by the STC to see the situation on the ground in Aden. What we found was a complicated tangle of separatist militias and politicians trying to consolidate control in a situation where alliances that may still exist on paper do not mean anything in reality. A situation that will make it even harder to bring an end to a war which has already killed an estimated 100,000 people and sparked the world’s worst humanitarian crisis.

Human cost of Yemen war laid bare as the death toll nears 100,000 Read more

After simmering hostility for years, the southerners and the government of the president, Abd Rabbu Mansour Hadi came to blows thanks to the Houthis: a devastating drone and missile attack on a military parade in Aden last month killed more than 30 STC soldiers, including an important senior commander, Munir Mahmoud, known as Abu al-Yamama.

Southerners accused the Hadi government of disregarding the threat posed to southern forces by the Houthis by failing to share intelligence. Demonstrations and four days of intense fighting followed, killing dozens and driving men loyal to Hadi out of Aden’s presidential palace, military bases and airport.

Facebook Twitter Pinterest Members of the STC (Southern Transitional Council) forces seen at a security checkpoint in the port city of Aden, on 16 September, 2019.

The fighting also called the stability of the Saudi-led coalition into question after the reaction of the United Arab Emirates, its second-biggest member. When Hadi’s Saudi-backed troops tried to launch a counterattack on 29 August, the UAE, which funds and arms the STC, launched airstrikes against them, killing about 40 soldiers and civilians.

Aden has been the interim internationally-recognised capital of Yemen since the Houthis forced Hadi to leave Sana’a in 2015, but in this new chapter of the war, the Yemeni government no longer has a presence there. The UAE strikes were condemned by Yemen’s information minister, Moammar al-Eryani, as a “treacherous” attack.

The port city of Aden is nestled in the natural harbour formed by the crater of a dormant volcano. Jagged mountains surround it, curled around the city like a sleeping dragon. The ocean blue of the STC flag, usually twinned with that of the UAE, is unmissable.

Facebook Twitter Pinterest A young boy plays in the sea as fishermen unload fish they caught earlier, at a fish market in the port city of Aden, on 8 September, 2019.

Many southerners say they were not happy with the creation of modern Yemen in 1994 and that the national government has consistently marginalised them.

Hadi, who became president in 2012 after Yemen’s longtime autocrat Ali Abdullah Saleh was deposed in the Arab spring, is seen as weak for his failure to stop al-Qaida from terrorising the south before help arrived from the UAE.

Adenis are proud of their distinct and cosmopolitan history: African, Chinese, Indian and Persian traders have all left their mark on the port city, as well as the Portuguese, Ottomans and the British, for whom there is a particular nostalgic fondness.

The golden days, however, are long gone. STC officials point out trappings of the British Empire, such as the only churches on the Arabian peninsula and a gate on the corniche built for the Queen’s visit in 1954, but they are all pockmarked with bullet holes and other damage from the war.

Aden’s luxurious beachfront hotels became the frontline in the battle to drive the Houthis out of the city in 2015. They tower above the sea, roofs caved in and their insides burnt out.

Facebook Twitter Pinterest Fadhel Mohammed al-Gade, member of the Southern Security Council, during an interview in his office, in the port city of Aden

“We have a difficult job to rebuild and make sure Aden is safe but we are ready to do so,” said Fadhel al-Gade, assistant secretary-general of the STC’s presidential council. “We are doing this for ourselves since others won’t.”

The STC may want independence but it is heavily reliant on its UAE patrons: one well-connected source in the city said the Emiratis came to the rescue with the airstrike against Hadi’s troops because the STC does not have the numbers to fight them off alone.

Facebook Twitter Pinterest The remains of beachfront hotels in the port city of Aden on 8 September, 2019.

The UAE’s aim in Yemen as well as elsewhere in the region is to curb the power of political Islamist groups such as Yemen’s Islah, which it sees as a threat, and to gain control of the crucial Bab el-Mandeb strait which connects the Red Sea to the gulf of Aden – one of the world’s most important oil shipping channels. A protectorate of sorts in south Yemen fits both bills.

Facebook Twitter Pinterest Two young boys selling balloons in the streets of Aden, on 8 September, 2019.

While the UAE announced it would withdraw the majority of its troops from the country in July, it still retains most of its sway on the ground through the STC and allied militias.

Maj Gen Ahmed Saeed bin Buraik said the UAE’s support in both the fight against the Houthis and Sunni extremists was welcome. As a former ruler of Hadramawt governorate, he was instrumental in driving al-Qaida out of the city of Mukalla before the STC was formed in 2017. “We won the support and trust of our friends in the UAE by making Yemen safer,” he said. “This is an equal partnership in every sense.”

The STC has strong local support in Aden and the surrounding Lahij, Abyan and Dhale governorates. It is very unlikey Hadi will try to retake the city again. In the rest of the south, however, the STC’s influence is not as strong, and a delicate negotiation must be struck with Hadi over central bank resources and maintaining government services: civil servants in Aden have not been paid their salaries for the past month.

“I don’t care who is in control,” said Umm Khaled Abdul Rahman, 43, as she shopped for supplies for the new school year with her twin daughters in the Krater district market. “I just need to be able to feed my family and the electricity to stay on for more than a few hours. That hasn’t happened in years.”

Hadi, in turn, needs the STC to continue allowing his government to access Aden’s airport. De-escalation talks in the Saudi city of Jeddah in the past few weeks have concentrated on the need for the STC, Hadi and Islah to continue cooperating in the fight against the Houthis on several different fronts and giving space to STC voices inside the government.

Facebook Twitter Pinterest Sultan al-Aradah, Governor of Ma’rib, pictured at his office in Ma’rib, Yemen on 3 September, 2019.

Hadi and Islah’s seat of power is in the process of shifting to the dusty central city of Marib, which is rich in oil and gas reserves. In the past few years, Marib’s governor, Sultan al-Aradah, has succeeded in turning Marib into a wild west-style boom town where displaced people from elsewhere in the country have found relative safety. On a brief visit en route to Aden, Aradah told the Guardian he believed a peaceful solution could be found to the new schism.

What happens next depends on whether the STC will try to shore up its position outside the city of Aden: most observers fear more clashes in Abyan province, the midpoint between Aden and government-loyal Shabwa.

The only certainty in the war-within-a-war is that more fighting will bring more civilian suffering.

Facebook Twitter Pinterest Ethiopian migrants are seen washing their clothes in a makeshift camp-reservation on the outskirts of Ma’rib, on 3 September, 2019.

Facebook Twitter Pinterest Children playing with water during a water distribution, in the al-Jafinah IDP camp in Ma’rib, on 3 September, 2019.

In the al-Jafinah displacement camp on the outskirts of Marib, Mohammed and Khadija Rubaiyah are doing their best to make a home in the heat and the sand.

They are from the lush green highlands of Hajjah in Yemen’s north, but were forced to leave in March after their area saw intense fighting between Houthi and government forces. They found an apartment in Aden, but during the recent clashes they said a fighter knocked on the door and told them that as a family from the north, they would be killed if they were not gone by the morning. The couple took their three small children and the little else they could carry and fled to Marib.

“This is no life for us,” said Mohammed, 35, who has to leave the family to return to work in Saudi Arabia soon. “But the war keeps going. I don’t know where we will end up next.”