By Michela Wrong

If nature abhors a vacuum, politics abhors a military standoff, especially between two nations in one of the poorest, most volatile and most strategically sensitive regions of the world.

And so there was much excitement when the government of Ethiopia announced on Tuesday that it would fully accept the ruling of an international tribunal in the country’s boundary dispute with Eritrea — some 16 years after the judgment was issued.

In 2002, a special international commission delineated the border between the two countries, as they had agreed in the peace deal that ended their 1998-2000 war. Demarcation on the ground was expected to start swiftly, allowing cross-border trade and cooperation to resume. But none of this happened.

Ethiopia accepted the ruling in principle but called for further dialogue and, crucially, kept its troops in place, including in what had been declared Eritrean territory. A few years later, the boundary commission dissolved itself, and in 2008, the United Nations peace monitoring force meant to oversee actual demarcation pulled out, its services unwanted.

What once seemed unsustainable — an indefinite state of neither peace nor war — became the norm. Both countries hosted guerrilla groups committed to overthrowing the other one’s government. They cynically fought a proxy war in neighboring Somalia. There were repeated flare-ups at their border, triggering apocalyptic predictions that Ethiopia and Eritrea were going to fight again, and next time to the bitter end.

Legally, Ethiopia clearly was in breach, having committed in the 2000 peace deal, like Eritrea, to uphold whatever decision the boundary commission issued. The United Nations, the European Union, the Organization of African Unity (now the African Union) and the United States had pledged to act as guarantors, and so were also in the wrong. Eritrea, for its part, had good reason as a fledgling country to crave international recognition for its borders.

But given the choice between a giant traditional ally led by an emollient prime minister and a tiny new-kid-on-the-block with a notoriously prickly president, the major Western powers opted to side with the bigger player — and all the more readily because it cast itself as an ally in the fight against Islamist terrorism.

So what prompted Ethiopia’s announcement this week? Age and sickness is one answer. Over the years, local analysts and former guerrilla fighters have told me that Ethiopia’s dispute with Eritrea was partly being kept alive by animosity between the two countries’ longtime leaders and their immediate entourages.

Years ago, Meles Zenawi and Isaias Afewerki, whose families both hail from the Tigray region that straddles the border, joined the forces of their rebel movements against Ethiopia’s Marxist dictator Mengistu Haile Mariam. They managed to oust him in 1991, paving the way for Eritrea’s formal independence from Ethiopia in 1993 — and then Mr. Meles’s rise to prime minister of Ethiopia and Mr. Isaias’s to president of Eritrea.

But rivalry and resentment simmered below the surface. In 1998, a dispute over the nondescript border village of Badme escalated into a war that would kill more than 100,000 people. Many Horn of Africa watchers predicted that relations between the two countries would only normalize once the two leaders quit the scene.

Mr. Isaias, 72, is still at the helm, although only last month he was reported to have left Eritrea for emergency medical treatment in Abu Dhabi. Mr. Meles died in 2012. His immediate successor, Halemariam Desalegn, resigned in February, seemingly overwhelmed by the task of running his discontented nation of some 105 million people. Mr. Halemariam’s fresh replacement, Abiy Ahmed — a spruce 41-year-old with a background in military intelligence — is a man in a hurry.

Abiy Ahmed, the newly elected chairman of the Ethiopian Peoples’ Revolutionary Democratic Front, in April

And with good reason. Ethiopia’s capital, Addis Ababa, may be booming, but so is unrest among a young population that scoffs at official 8-to-10 percent annual growth rates, accuses Mr. Meles’s party — which long dominated the ruling coalition — of ethnic chauvinism and corruption, and chafes at government repression. Foreign exchange reserves are running low; the national debt is climbing. Ethiopia has lived through coups and popular revolutions before, and in recent years the Oromo, who make up the country’s biggest ethnic group but have long been marginalized, have been at the forefront of protests. Appointing Mr. Abiy, an Oromo, as prime minister was a smart survival move; the Ethiopian government realized that real change was required.

In recent years, Western diplomats have grown more and more worried that an increasingly isolated Eritrea, resentful at its treatment by the international community and routinely dubbed a “pariah state” for its domestic human rights record, might come to be seen as an attractive destination by jihadists spilling out of nearby Yemen, Syria and Iraq.

Any such infiltration would be particularly unwelcome given rising geostrategic interest in the Horn of Africa over the last decade and a half. The Red Sea has quietly become one of the world’s most important waterways, with foreign military assets and investment pouring into the region’s ports, railways, airports and roads. Djibouti, landlocked Ethiopia’s de facto outlet to the sea, now hosts troops from the United States and France, but also China, Germany and Japan. The United Arab Emirates’ military operates out of the ports of Assab in Eritrea and Berbera in Somaliland.

For such players, the stalemate between Eritrea and Ethiopia was becoming politically and financially untenable. It is probably no coincidence that Ethiopia’s shift about the boundary this week follows a visit to the region in late April by Donald Yamamoto, the United States’ acting assistant secretary of state for African affairs.

While cheering Mr. Abiy’s declaration about the border, diplomats are stunned by the rat-a-tat pace of his sudden departures from old practice. First came the release of the opposition leader Andargachew Tsige, a bête noire of the Ethiopian government, along with several hundred political prisoners. Then the state of emergency was lifted. After that it was announced that state-owned enterprises would be opened to private investment.

“This is breathtaking stuff,” said a diplomat who has spent years shuttling between the region’s capitals. “The pace of change is incredible, and the prime minister needs every bit of support from the international community if he is to push this through.”

And yet the much-awaited, much-desired normalization of relations between Ethiopia and Eritrea could prove more destabilizing to the Horn of Africa in the long term than its cold war ever was.

For all of Mr. Isaias’s complaining about Ethiopia’s refusal to honor the boundary decision, that reluctance has served him well: It has allowed his control-freak regime to keep running Eritrea along the militaristic lines he and his movement established in the bush during the fight for independence. His government could invoke the threat of an imminent invasion to justify its refusal to implement the 1997 Constitution, allow opposition parties, stage multiparty elections or tolerate a free press.

Mr. Isaias’s insistence that all Eritreans’ first duty is to protect their country has kept much of the nation’s youth trapped in open-ended military service. The policy has crippled the economy, leaving Eritrean farms and businesses bereft of labor. It has also been massively unpopular, including within the military itself. In 2013, Mr. Isaias survived a coup attempt by junior army officers.

At the same time, indefinite forced conscription has allowed Mr. Isaias to pre-empt the kind of mass protests that roiled northern Africa during the Arab Spring. Eritreans who can’t stand living conditions in Eritrea flee rather than rebel. In one of the saddest exoduses in contemporary African history, tens of thousands of them have risked their lives heading for the Mediterranean Sea and then trying to cross it. Many have drowned; others have wound up rotting in Libyan prisons or being held hostage by human traffickers in the Sinai Peninsula.

If Ethiopia does withdraw its troops from the Eritrean territory it still occupies, a key excuse for Mr. Isaias’s iron rule will be removed.

His admirers hope that he would grab any historic opportunity for real peace with Ethiopia to display once again the visionary leadership that defined him as a freedom fighter and reset his management of the country.

His critics, who see him as incapable of shifting gears, believe the sustained bluff that was mass conscription may have just been called. If they are correct, Ethiopia’s recent peace overture could actually make the region more, not less, volatile.