The latest violence — increasingly being known as the civil war within Yemen’s civil war — pits southern separatist militias against other forces aligned with the government of President Abed Rabbo Mansour Hadi. Tensions surfaced more than a year ago between the allies, but in August they boiled over into the worst fighting so far between them.

Both sides are aligned with a regional coalition of Sunni Muslim countries, led by Saudi Arabia and the United Arab Emirates, that joined the war against the northern Shiite rebels known as Houthis in March 2015. The coalition’s ostensible goal is to restore Hadi’s government and prevent the Shiite theocracy of Iran from gaining regional influence through its alliance with the Houthis.

The war in the Middle East’s poorest country has caused poverty and illnesses to spiral. ­Yemen faces what the United Nations has described as the world’s worst humanitarian ­crisis.

In early August, clashes erupted in the strategic southern city of Aden between the southern separatists, backed by the UAE, and forces loyal to Hadi, which are backed by Saudi Arabia. The trigger was a missile attack on a military parade that killed dozens of separatist fighters. It was partly blamed on Islah, an Islamist party aligned with the Yemeni government.

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The separatists seized control of Aden, the seat of the exiled Yemeni government, deepening the rift between Saudi- and UAE-backed factions. In recent weeks, despite calls for a cease-fire, the fighting has expanded to other southern provinces. Saudi Arabia and the UAE have carried out airstrikes in support of their Yemeni proxies.

On Aug. 28, Yemeni government forces entered Aden and claimed to have retaken the city. But the next day, separatist forces said they had taken it back. The heavy fighting has caused a new wave of civilian casualties in the city, humanitarian groups said.

The fighting in southern Yemen could reverberate beyond that region in at least a half-dozen ways:

Yemen could fracture

The tensions in the south have roots in decades of friction between the north and south. Yemen was once two countries — North Yemen and South Yemen — until unification in 1990. Many southerners have remained suspicious of the northerners, who have ruled the country for decades, accusing them of politically and economically marginalizing them.

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The separatists want to split Yemen’s south from its north again. Even if that doesn’t happen, analysts say that after more than four years of conflict and mistrust, Yemen is unlikely to emerge with a strong central government. Already, different centers of power have formed in the country. Some analysts say a federalist system where the country is carved up into autonomous regions loosely aligned with the center is the best ­anyone can hope for.

Regional powers divided

Even as Saudi Arabia and the UAE are aligned against the Houthis and Iran, they have differing visions for Yemen’s ­future. The UAE and the ­separatists are wary of Hadi’s alliance with Islah, whose members include Ali Mohsen al-Ahmar, Hadi’s vice president. The UAE leadership views Islah as a threat because of its links to the Muslim Brotherhood, a regional Islamist movement that the Emiratis and their allies see as radicals. The Saudis, however, view Islah as playing an essential role in Yemen’s war and future politics.

While the Saudis view their role in Yemen as a deterrent to Iran’s ambitions, the Emiratis have an additional goal: gaining influence in the southern Arabian Peninsula and the Horn of Africa, which lie next to lucrative and strategic shipping lanes in the Red Sea and Indian Ocean. Analysts say Aden, with its port near the Red Sea, is a valuable economic prize for the UAE.

To assert control, the UAE has backed influential southern politicians and has armed and financed southern separatists, tribesmen and other militias, including one southern commander who is on the U.S. sanctions list because of alleged links to al-Qaeda and the Islamic State.

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In June, other cracks emerged in the Saudi-led coalition. The UAE said it was pulling its ground forces from southern Yemen, a move some analysts interpret as a signal that the Emiratis no longer agreed with the Saudis’ hard-line approach to Iran.

The recent clashes have further frayed the unity of the coalition. Saudi planes have bombed separatist positions, and UAE fighter jets have bombed Yemeni government forces. The question on many minds: Will the divisions in Yemen harm the relationship between Saudi Arabia and the UAE in other regional matters?

Opening for al-Qaeda, ISIS

The south has long been a haven for al-Qaeda’s Yemeni affiliate, al-Qaeda in the Arabian Peninsula, or AQAP. Considered by U.S. officials to be the terrorist network’s most dangerous branch, AQAP has targeted the West several times, including an attempted bombing of an American passenger jet landing in Detroit on Christmas Day 2009. In 2000, al-Qaeda bombed the USS Cole in Aden harbor, killing 17 U.S. sailors. A campaign of U.S. airstrikes helped weaken AQAP, but it remains a powerful force in the south. An Islamic State affiliate has also emerged in recent years.

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In August, both groups saw opportunity in the violence and chaos shattering Aden. According to Elisabeth Kendall, a Yemen scholar at Oxford University, AQAP staged nine attacks on the UAE-backed separatist forces. The Islamic State launched at least two attacks in Aden, the first time the group has struck the city in more than a year. “AQAP and ISIS are already taking advantage of the increasing security vacuum in Yemen’s south,” Kendall said.

Threat to global shipping

Aden lies strategically near the mouth of the Red Sea and along shipping lanes from the Strait of Hormuz, near Iran. These are some of the world’s most vital commercial routes, being used daily by oil tankers from Europe and Asia.

Disruptions from clashes, airstrikes or bombings — or if militants assert control of the south — could curtail oil supply and raise global fuel prices. All sides, including the Houthi rebels, have the capability of attacking ships at sea. Such attacks have occurred during the war.

Disruption of aid

Aden is a key port of entry for humanitarian aid and commercial goods, not only for the south but also for many areas in the north. According to the United Nations, about 80 percent of the population — 24 million people — requires some form of humanitarian or protection assistance, including 14.3 million who are in dire need. The clashes in Aden have disrupted the work of dozens of aid groups.

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“Since August 28, Aden airport has been temporarily closed, and flights suspended,” Save the Children’s acting country director in Yemen, Jason Lee, said in a statement. “Moreover, the port of Aden is vital for the import and distribution of basic supplies and services across Yemen, and insecurity threatens its operation. It is highly concerning that vital lifelines, both by air and sea, are at risk of being cut off.”

Need for a broad peace deal

Efforts to end Yemen’s conflict are particularly focused on the western port city of Hodeida. A fragile U.N.-brokered cease-fire is still in place. U.N. officials and aid workers hope that if the peace lasts in Hodeida, it could have a ripple effect across the country. But the tensions in the south could torpedo efforts in Hodeida as the coalition’s allies battle each other, allowing the Houthi rebels opportunities to seize territory. The southern animosities also show that the grievances fueling Yemen’s multiple internal conflicts may not be addressed by bringing peace to one city.

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