The rapid takeover of a major city in Sudan’s agricultural breadbasket by the paramilitary group Rapid Support Forces on Tuesday sent shockwaves across the country, raised doubts about the power of its rival – the Sudanese army – and ushered in a new and potentially deadlier phase in the eight-month civil war devastated one of Africa’s largest countries.

It took the paramilitary group Only four days to conquer the city, Wad Madani, where tens of thousands of people had fled from the capital Khartoum, about 100 miles northwest, when the war began in April. The fall of Wad Madani has once again forced them to flee and dealt a serious blow to the reputation of the army that had promised to protect them.

“Depression is an understatement of how we feel,” said Omnia Elgunaid, a 21-year-old international relations graduate who fled Wad Madani on Tuesday to a village further south. “People are devastated because they now feel unsafe across the country.”

The army confirmed in a statement announced on Tuesday evening that it had withdrawn from the city and – in a highly unusual move – said it had launched an investigation into why the defeat occurred.

The war has already killed at least 10,000 people, although Sudanese health workers and United Nations officials say that is a significant underestimate.

According to the United Nations, about 300,000 people have fled Wad Madani in recent days. Sick and hungry, many of them left the city on foot and walked for hours to neighboring states, dragging suitcases and sheets with their meager belongings behind them.

Aid agencies have largely suspended operations in Wad Madani and wider El Gezira state, and the UN has relocated its staff there quieter areas in the east of the country or across the border into South Sudan. Aid workers, who have made the city a hub for their efforts, are concerned about the possibility of looting of aid supplies and warehouses.

“The Sudanese people have endured eight months of horror and the humanitarian situation continues to deteriorate,” said Sofie Karlsson, spokeswoman for the U.N. Office for the Coordination of Humanitarian Affairs in Sudan. “When the only option is to set out on foot with what you can carry, you know conditions have reached rock bottom.”

Amid the tumult, much focus was on the army’s battlefield tactics and its chief, General al-Burhan.

In Rufaa, a town about 30 miles north of Wad Madani, the army blocked paramilitary forces from crossing a bridge with a shipping container, a desperate deterrent measure that failed to stop their advance, residents said.

Experts say part of the reason for the army’s recent setbacks is due to its history.

Under former dictator Omar Hassan al-Bashir, the Sudanese army largely outsourced the task of ground combat to tribal militias like the Janjaweedthe hated group that terrorized the Darfur region in the 2000s and later became the Rapid Support Forces.

Now that the army has to fight a bitter war across a vast country, its weaknesses are quickly becoming apparent, said Alan Boswell, Horn of Africa director of the International Crisis Group.

“It was a highly politicized army, people were often promoted based on ideology and nepotism. It became very corrupt,” Mr. Boswell said. “The Army has never had to fight a war like this before and has proven itself unfit for purpose.”

The fall of Wad Madani shows that failures extend to the top of the army, said Kholood Khair of Confluence Advisory, a Sudanese research group.

“Something has gone terribly wrong in the leadership of the Sudanese armed forces,” she said. “This is something even some of them don’t understand.”

The capture of Wad Madani could pave the way for the paramilitary group to launch new attacks on other major cities, including Gedaref in the east and Kosti in the south.

Buoyed by their success, the paramilitaries may now be trying to spark an insurgency within the army, analysts say.

In a social media post on Tuesday, the RSF commander stated: Lt. Gen. Mohamed Hamdan, said His forces would not “become an alternative army” – a remark that many observers saw as an attempt to undermine the army chief, General al-Burhan.

Whether General al-Burhan can keep his job, Mr. Boswell said, depends on whether other Sudanese generals are willing to make a potentially destabilizing leadership change in the middle of the war and risk splitting their own ranks.

General al-Burhan is the key interlocutor with the army’s foreign backers, Mr. Boswell said, adding: “Other generals may need to weigh whether his removal could harm those ties.”

As the conflict enters a new phase, the risk of foreign interference also increases, experts say. These include neighboring countries such as Eritrea, its autocratic leader met in September and November with the Sudanese army chief.

The United Arab Emirates has provided weapons and medical support to the paramilitary forces. They deny these allegations. A destabilized Sudan would also be worrying for Saudi Arabia, which lies across the Red Sea.

A major regional conflict would be “a nightmare scenario not just for Sudan but for the world,” said analyst Ms. Khair.

Many Sudanese hope that they can find food and shelter for the time being. On Wednesday, Ms Elgunaid said she woke up with a fever but none of the pharmacies in the village where she was staying were open. Phone and internet connections were slow and many people were still sleeping outdoors, she said.

“We have no idea what we’re going to do next,” she said. “We all feel trapped.”

Declan Walsh Contribution to reporting from Kinshasa, Democratic Republic of Congo.