NAIROBI, Kenya – When it comes to mountain warfare, the people of Tigray — an ancient kingdom in the far north of Ethiopia, spread across jagged peaks and lush farmland — have decades of hard-won experience.

Tigrayan fighters led a brutal war through the 1970s and ’80s against a hated Marxist dictator of Ethiopia, whom they eventually toppled in 1991, becoming national heroes. For most of the next three decades, Tigrayans ruled Ethiopia.

But after Abiy Ahmed, a peace-talking young reformer, came to power as prime minister in 2018, he brusquely sidelined Tigray’s leaders. Tensions exploded violently on Nov. 4, when Abiy launched military strikes in Tigray.

Now Tigray is once again at war, fighting the federal government. But this time the risks could be even bigger: the potential fracturing of Ethiopia and the upending of the entire Horn of Africa.

The battle pits the nation’s army and Abiy, an internationally feted winner of the Nobel Peace Prize, against the ruling party of Tigray, which commands a large force of well-armed and experienced fighters who know their own mountain terrain well. Already the conflict has escalated at alarming speed with intense fighting that has involved airstrikes and artillery barrages, sent thousands of civilians fleeing across borders — some in boats or even swimming — and led to reports of civilian massacres.

With such intransigent foes, analysts predict a potentially long and bloody fight that is already spilling over Ethiopia’s borders.

On Friday, Tigray launched rockets at two airports in neighboring Amhara Province and on Saturday said it had fired a volley of rockets at the main airport in Ethiopia’s neighbor Eritrea, which Tigray accuses of siding with Abiy.

The rush to war has exacerbated ethnic divisions so badly that Friday it prompted warnings of potential ethnic cleansing and even genocide.

“The risk of atrocity crimes in Ethiopia remains high,” said Pramila Patten, the United Nations’ acting special adviser for the prevention of genocide, and Karen Smith, the special adviser on protecting civilians, in a joint statement.

Until recently Ethiopia, a close U.S. military ally, was seen as the strategic linchpin of the volatile Horn of Africa. But with its brewing civil war spilling into Eritrea, refugees streaming into Sudan and Ethiopia’s peacekeeping mission to Somalia now under strain because of its domestic turmoil, analysts worry that Ethiopia could destabilize the region.

The dispute between Abiy and the Tigrayans goes back to the early days of his term as prime minister two years ago.

He moved quickly to shake up the country after decades of iron-fisted rule under the Tigray People’s Liberation Front. Political prisoners were freed, exiled dissidents were welcomed home and Abiy promised free elections and press freedom.

Those rapid, wide-ranging reforms, which eventually helped Abiy win the Nobel Peace Prize, were a pointed repudiation of the Tigrayan old guard.

With phone and internet connections cut off, it’s hard to know exactly what is happening in Tigray. By Sunday at least 20,000 Ethiopian civilians had fled into Sudan, a refugee stream that the United Nations fears could quickly become a flood. Sudan says it is preparing for up to 200,000 refugees.