AL-FASHAGA, Sudan — This fertile borderland between two rivers has long held the key ingredients for a war.

For decades, a precarious status quo prevailed here between Sudan, which owns the land according to a century-old treaty, and Ethiopia, which has occupied it while its citizens tilled the fields of sesame, sorghum, sunflower and cotton.

But Ethiopia’s sudden descent into civil war in its Tigray region has upended a delicate web of regional political equations, sending ripple effects across this corner of Africa, and bringing Ethiopia and Sudan to the brink of a territorial war over this disputed area, known as al-Fashaga. Military and government officials on both sides, as well as independent analysts, said they worry such a war would quickly escalate into a much broader regional conflict.

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Sudanese officials also accused Ethiopian Prime Minister Abiy Ahmed of attempting to force Sudan’s hand over al-Fashaga by holding up negotiations over the filling of a mega-dam Ethiopia is building near the two countries’ border. Filling the dam without an agreement, they claimed, could imperil drinking and irrigation water for half of Sudan’s population.

“Abiy is underestimating the risk to the region his actions create. Does he believe that he can put the water and livelihoods of 20 million Sudanese at risk and that we would accept that?” said Yasir Abbas, Sudan’s water minister. “Regional stability is at stake. Any kind of conflict between us will immediately spread to a wider region — the Red Sea, the rest of the Horn of Africa.”

On a recent trip with Sudanese forces to the front line, a major deployment of military and paramilitary troops was visibly underway. Through binoculars, Sudanese officers eyed Ethiopian settlements and fortifications in parts of al-Fashaga they had not yet retaken.

Despite blisteringly hot weather, the Sudanese side buzzed with activity. In just a couple months, Sudan has built an extensive road network in areas it now controls. Locals had been pulled into the effort, working on construction, cooking for soldiers and even patching up their uniforms. But nervousness lay beneath the industriousness.

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Since retaking much of al-Fashaga early this year, the Sudanese military has quickly set about building infrastructure in the region. (Max Bearak/The Washington Post)

An internal United Nations map published last week and reviewed by The Washington Post shows multiple deployments of Ethiopian National Defense Forces in al-Fashaga, and at least 16 sites of clashes, including an attack on civilians. The Sudanese military says more than a dozen of its soldiers have been killed, and Sudanese residents said armed Ethiopian farmers had slaughtered at least a dozen unarmed civilians.

Ethiopia’s military has not announced its casualties, but Gizachew Muluneh, an Ethiopian regional government spokesman, said nearly 2,000 civilians had been displaced. He called the Sudanese deployment an invasion and said al-Fashaga belonged to Ethiopia’s Amhara region, which he represents. Abiy has made statements supporting that claim.

According to a 1902 agreement between Menelik II, then Ethiopia’s emperor, and Sudan’s British colonial overlords, al-Fashaga is Sudanese land. In the mid-1990s, however, while Sudan’s military was fighting numerous domestic wars, Ethiopian soldiers and farmers moved into the area and established settlements behind military lines.

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Sudanese army soldiers pose with a water tank painted in the colors of the Ethiopian flag. It was recovered from what the soldiers said was an Ethiopian military base in al-Fashaga. (Max Bearak/The Washington Post)

“Before, it was an irregular occupation, but since Abiy came to power, they have begun to claim, ‘This is our land,’ ” said Col. Abadi el-Tahir, a field commander in the Sudanese military. “They even dug up cemeteries and brought dead bodies to rebury them in Sudan in an attempt to claim that this land is theirs, as if they have been there for generations.”

“We are working day and night to take it back,” he added. “Al-Fashaga is not totally under our control yet, but almost.”

Sudan retook most of al-Fashaga after Ethiopian soldiers and Amhara militias were deployed to fight in Tigray.

“Up until the war in Tigray began, the situation was essentially an Amhara occupation of al-Fashaga,” said Maj. Gen. Ibrahim Gabir, one of 11 members of Sudan’s so-called sovereign council that presides over government decisions.

“As they became occupied with the war in November, we were able to retake the area with fewer casualties. But recently the Amhara militias came back and killed so many of our people, robbed so many,” he said. “Ethiopia is not controlling these militias, so of course we see it as supporting them. We do not want to be drawn into a reckless war.”

Abiy’s increasing reliance on Amhara support for the war in Tigray is a fundamental driver of conflict in al-Fashaga, analysts said. Even though Abiy claimed victory in the war in November, fighting has continued, according to the United Nations and journalists who have gained access to the region.

The conflict has drawn the attention of the United States. This week, Delaware Sen. Christopher A. Coons traveled to Addis Ababa to meet with Abiy to discuss the “deteriorating situation in the Tigray region and the risk of broader instability in the Horn of Africa.” In congressional testimony earlier this month, Secretary of State Antony Blinken called for an investigation into what he called “acts of ethnic cleansing” in Tigray.

Amhara militias as well as troops from neighboring Eritrea have become essential to Abiy’s efforts to subdue rebels in Tigray — a war that has already cost thousands of lives and left millions in dire need of humanitarian aid.

In return for sending reinforcements to Tigray, Amhara leaders including Abiy’s intelligence chief and foreign minister have pressed Abiy to give major concessions to the region, including retaliation at Sudan’s border deployment, according to Nizar Manek, an independent analyst who is writing a book about the region. On Tuesday, Muluneh, the Amhara regional spokesman, said the federal government was preparing to transfer a huge swathe of Tigray’s land into Amhara administration, a form of retribution that Amhara hard-liners have long demanded.

“Under Abiy, Sudan rightly sees Ethiopia’s foreign policies as having tribalized on Amhara lines,” said Manek. He noted that the Amhara position is even further strengthened by coming national elections in June, for which the region has become a kingmaker.

That sudden gaining of leverage among Amhara hard-liners has made Abiy less able to compromise on either al-Fashaga or the dam, said Abbas, the water minister.

“Ethiopia is linking the dam issue with the border issue even though it has no right to do so. But Dr. Abiy is in a corner and must have Amhara support for his war in Tigray,” Abbas said. “The rise of the Amhara in Ethiopia’s politics is threatening the future of relations between our nations — not countries, but nations, peoples with literally thousands of years of shared history.”

Abbas said that Sudan’s government sees Ethiopia’s filling of the nearly complete dam during the coming June-July rainy season as an existential threat. During an initial filling that took place last year, he said millions of Sudanese were deprived of water for three days, and parts of the Blue Nile were shallow enough to walk across.

High-ranking Sudanese officials painted the dam and border disputes as easy to iron out, if not for Ethiopia’s increasingly uncompromising internal politics. On the border, joint demarcation was still on the table, Gabir said. Ethiopian farmers could be leased land in al-Fashaga, for instance.

“We have been asking for Ethiopia to cooperate in demarcating the border for decades,” he said. “We have said please, please, please and please again. They are not interested. Why? Because al-Fashaga is very good land. The Amhara will not give it up just like that.”

For now, though, Sudanese forces say they have retaken much of al-Fashaga’s farms, though at least three large Ethiopian settlements remain guarded by Amhara militias.

With their recent advances, Sudanese locals are rekindling hopes that they will be able to reclaim farms in the region that belonged to their parents and grandparents.

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Residents of the village of Wad ‘Arud on the edge of al-Fashaga convene to discuss plans to reenter the region ahead of the coming rainy season. (Max Bearak/The Washington Post)

“God willing, for the first time in 25 years we will cultivate that land between the two rivers,” said Abbas el-Tayyib, the mayor of the town of Qurayshah, where many farmers from al-Fashaga relocated after the Ethiopian occupation.

He, like many, remembers a time when Ethiopian farmers peacefully came to work as laborers on Sudanese farms. While their bitterness toward the Ethiopian government was sharp, locals commonly referred to Ethiopian farmers as their brothers and said the land was fertile enough to share.

“We remember exactly which fields are whose, even if the Ethiopians have changed the landscape,” said Ali Mohamed Ali, chief of the village of Wad ‘Arud. “If it rained tomorrow, we would go plow. We are ready.”

With Ethiopia’s next dam filling just months away and the massing of troops in al-Fashaga, however, the chances that any new clashes could quickly escalate into open war are high. Sudanese officials said all options to regain total control of al-Fashaga remain on the table.

“We both have serious, sensitive, internal issues to deal with, and we will respect Ethiopia’s right to deal with their own,” Gabir said. “But if they come into al-Fashaga, we will kill them, yes.”

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Residents of Wad ‘Arud walk toward a meeting place where they discussed plans to reenter the region. (Max Bearak/The Washington Post)