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(CNN) —  

Last Wednesday, fighting broke out in the coastal city of Aden, the temporary seat of the internationally recognized government of Yemen.

But this time it was not the Saudi-led coalition battling Iranian-backed Houthi rebels, as has usually been the case in the country’s past four years of brutal civil war.

Instead, factions within the coalition took arms against each other, killing dozens and threatening the alliance between Saudi Arabia and the United Arab Emirates.

By Sunday, the separatist Southern Transitional Council (backed by the UAE) was effectively in control of Aden and its nominal ally, the government of exiled President Abd Rabbu Mansour Hadi (backed by Saudi Arabia), was nowhere to be seen.

What’s happening in Aden?

In four days of days of fighting between the southern separatists and the president’s forces, 40 people were killed and 260 injured.

As the separatists made gains, the Saudi-led coalition intervened to defend the government, hitting an empty field at the presidential palace after the separatists took control of it.

The southerners took the airstrike as a warning shot and left the palace, but remained in control of Aden.

Yemen’s interior minister, Ahmed al-Maysary, called it a “successful coup”, as he conceded defeat in a video before joining the rest of Hadi’s government in the Saudi capital, Riyadh.

Relative calm has returned to the city, following days of street battles that trapped civilians in their homes.

Why are they fighting?

How far do you want to go back? The STC has long sought southern independence. They put these aspirations on hold when Houthi rebels took over the capital, Sanaa, in 2014.

STC then joined the Saudi-led Coalition and agreed to work under Hadi’s government. They take credit for many of the coalition’s victories over the past four years, but this hasn’t translated into proper inclusion in the government or peace talks.

They also regularly complain about government corruption and military failure.

More recently, the STC has accused Hadi’s Presidential Guard of collaborating with Houthis in an attack that killed one of their military leaders. They also accused them of attacking his funeral.

The government said it was defending state institutions from thousands of protesters. Before long, fighting spread to the streets of Aden, despite calls for calm by regional leaders.

Is this a proxy war between the UAE and Saudi Arabia?

Sarcastically, Ahmed Al-Maysary, Hadi’s interior minister, congratulated the UAE for victory as he conceded defeat – a brief moment of candor that revealed the possibility of a proxy war between the UAE and Saudi Arabia.

The two sides have longstanding political differences. Riyadh is the de facto center of Yemen’s government, and Hadi and his ministers spend most of their time in Saudi Arabia. Meanwhile, the United Arab Emirates has empowered the separatists, relying on them, and others, to win battles on the ground.

01:03 - Source: CNN
Official's impassioned response to aid in Yemen

The UAE is at odds with Saudi Arabia over the Hadi government’s inclusion of members of the Islah political party. The party is known for its connection to the Muslim Brotherhood, an organization that the UAE labels as terrorist and whose rise it has worked to cap over the past decade.

On Monday, Saudi and Emirati leaders met to discuss Yemen. But pictures of warm receptions don’t reveal the impact of recent infighting on their alliance.

What does this mean for the war in Yemen?

Whether you think the conclusion of Yemen’s prolonged war will come through military might or peace negotiations, the Aden conflict complicates everything.

A fractured coalition is not enough to tip the balance in anyone’s favor. It could even mean fighting will drag on longer, with multiple sides involved.

Hadi’s legitimacy and credibility will be hugely undermined if he can’t speak on behalf of all he claims to represent.

And on the humanitarian side, any interruption to a vital port like Aden complicates the work of aid missions responding to the world’s worst humanitarian crisis.

But there is a silver lining. This could be a wake up call to Saudi Arabia, the UAE and other parties that any sustainable solution requires addressing everyone’s grievances.

The December Stockholm Agreement focused on short term goals, but negotiations didn’t include the south’s immediate concerns or future aspirations.

As relative calm returns to Aden, and both sides agree to Saudi-sponsored talks, there is still time to reconsider old ways.