Nega remained vague about his past. But students curious enough to Google him would discover that the man who stood before them, outlining development policies in sub-Saharan Africa, was in fact intimately involved in the long-running hostility between Ethiopia and neighboring Eritrea, a conflict that has dragged on for half a century. By the start of the millennium, its newest incarnation, a border war over a patch of seemingly worthless ground just 250 square miles in size, devolved into a tense standoff, with the two nations each massing along the border thousands of troops from both official and unofficial armies. One proxy army fighting on the Eritrean side, a group of disaffected Ethiopians called Ginbot 7, was a force that Nega helped create, founding the movement in 2008 with another Ethiopian exile, Andargachew Tsege, in Washington. The Ethiopian government, which had previously detained Nega as a political prisoner for two years in Addis Ababa, now sentenced him to death in absentia. Bucknell students who did learn about their teacher’s past were thrilled. “It made his classes exciting,” Rickard says.

In Ginbot 7, Tsege served as the political leader based in Eritrea; Nega was the group’s intellectual leader and principal fund-raiser, collecting money from members of the Ethiopian diaspora in Europe and the United States. That all changed one day in June 2014, when Tsege, known to everyone as Andy, made a brief stopover in Sanaa, the capital of Yemen, on his way to Asmara, the capital of Eritrea. As he sat in the airport transit lounge, waiting to board his flight, Yemeni security forces, apparently acting in collusion with Ethiopian intelligence, arrested him and put him on a plane to Addis Ababa, where he was paraded on state television and currently faces a death sentence.

Days after Tsege’s arrest and extradition, Nega volunteered to replace him in Eritrea. “Was I going to remain an academic, sitting in an ivory tower criticizing things?” he told me. “Or was I going to do something as an engaged citizen?” Nega put his house up for sale and took an indefinite leave of absence from the university. It was an extended sabbatical, he told his colleagues. Only a handful of close friends, his wife and his two sons knew the truth.

On a hot July afternoon in 2015, Nega packed a suitcase, bade his wife farewell and was driven by comrades to John F. Kennedy International Airport. He carried a laissez-passer from the Eritrean government, allowing him a one-time entry into the country. Nega was heading for a new life inside a destitute dictatorship sometimes referred to as the North Korea of Africa; the regime was notorious for having supported the Shabab, an Islamist terrorist group in Somalia, and for a military conscription program that condemns many citizens over age 18 to unlimited servitude. Nega also believes he has drawn the scrutiny of the Obama administration and was worried about being stopped and turned around by Homeland Security. It wasn’t until the wheels on the EgyptAir jet were up and he was settling into his seat over the Atlantic Ocean, bound for one of the most isolated and repressive nations on Earth, that he was able to relax.

The lights cut out above Nega one chilly night this July, and the rebel chief sat in darkness in a bungalow in Asmara, Eritrea’s 7,600-foot-high capital. Nega had spread a map on a coffee table, and he was showing me the route for a clandestine mission that he planned to undertake the following morning. At dawn, he and a comrade would drive 300 miles southwest to the mined, militarized border between Eritrea and Ethiopia to rendezvous with intelligence sources at a rebel base camp. His contacts were smuggling across the border “highly sensitive information” about Ethiopian troop positions and about the strength of resistance cells inside Ethiopia, whom Nega was hoping to link up with his own fighters on the Eritrean side of the border.

“They’ve got documents, and they insist on handing them over only to me,” Nega told me. “When there is sensitive material, they first want me to see it and then filter the information to the rest of the organization.” Nega, a burly, balding 58-year-old with a rumpled facade and an appealingly unassuming manner, rubbed his forehead as the lights flickered and then returned. In recent years, Ginbot 7 has grown, and it is now guided by an 80-member council of representatives spread around the world. As commander, Nega oversees several hundred rebel fighters in Eritrea as well as an unknown number of armed members inside Ethiopia who carry out occasional attacks in the movement’s name. During his frequent visits to the front lines, he spends his time meeting with fellow commanders, observing training and — ever the professor — leading history and democracy seminars using chalk and a blackboard in a “classroom” in the bush.

Nega turned back to the map and traced a straight line leading to the Tekeze River, the westernmost border between Ethiopia and Eritrea. The stream was a main crossing point for Ethiopian Army deserters fleeing to the rebels, and in recent weeks it had come under threat from advancing Ethiopian troops. “They are moving a sizable force into this area, because we are their main target now,” he said, referring to Ginbot 7, now known as Patriotic Ginbot 7. “And they are pushing a large part of their army, artillery and tanks into this zone. They haven’t started shelling us yet.”

The two nations, now ferocious enemies, were once joined. Eritrea, an Italian colony from 1890 until 1941, was annexed by Ethiopia after World War II; it took a three-decades-long war for the Eritreans to finally liberate themselves, in 1991. The neighbors remained at peace until 1998, when a simmering dispute over the Yirga triangle, a piece of rocky land along the border that had never been clearly demarcated in colonial maps, exploded into two years of tank and trench warfare in which 100,000 died. Today, despite a United Nations-supervised mediation that awarded the disputed territory to Eritrea, Ethiopia continues to occupy the border village Badame. Tens of thousands of troops face each other across a landscape of mines, bunkers, sniper posts and other fortifications.

Violence on the border, while infrequent, can be both sudden and brutal. In mid-June, according to the Eritrean government, Ethiopia launched a full-scale attack along the frontier at Tsorona, the first major incursion since 2012, possibly in retaliation for attacks on its forces by Ginbot 7. Eritrea claimed that it had killed 200 enemy soldiers and wounded 300, though Ethiopia downplayed its losses. “They almost always deny it,” Nega told me. “As far as the Ethiopian government is concerned, nobody ever dies.”

Ethiopia, while an American ally and an economic leader by African standards, is notoriously repressive. The minority Tigrayan regime has jailed hundreds of bloggers, journalists and opposition figures, keeping itself in power by intimidating political opponents, rigging elections and violently putting down protests. Since November of last year, according to Human Rights Watch, state security forces killed more than 400 protesters in the Oromia region, which surrounds Addis Ababa. Protests have recently spread to the Amhara region, as well; in August, security forces shot dead roughly 100 demonstrators and injured hundreds more. Thousands of Oromos, a minority group that makes up about a third of the population, have been jailed without trial on suspicion of supporting the Oromo Liberation Front, a secessionist group. The Ethiopian marathoner Feyisa Lilesa, who won the silver medal at the Olympics this year, drew global attention to the government’s abuses when he held his crossed arms over his head at the finish line in solidarity with his fellow Oromos; he says he fears returning home and is seeking political asylum.

Across the room in Nega’s bungalow, four fellow rebel commanders, all members of the Ethiopian diaspora, were finishing their supper. The men tore off pieces of injera and dipped the bread into a thick sauce called shiro, washing down the meal with bottles of the local Asmara beer. Esat, an Ethiopian opposition satellite channel broadcast from Europe and the United States, played softly on a television in the corner. The men were part of a revolving contingent of commanders who returned to Asmara from time to time to check their email and escape the primitive conditions in the bush. “We are five right now,” Nega said, introducing me to his comrades from Dallas; Arlington, Va.; Calgary, Canada; and Luxembourg. “Another, from the United Kingdom, is returning here tomorrow morning. We’ll be six when he comes. Last week we were eight — at one point we were 11.”

The house also serves as an infirmary for rebels who become ill or are wounded in combat, and it provides a temporary sanctuary for Ethiopian Army defectors who cross the front lines. One recent arrival was a former Ethiopian Air Force officer, an Oromo who had traveled north 42 hours by bus and on foot, then swum across the Tekeze River to Eritrea. He made the decision to defect while sitting in an Addis Ababa jail cell on “false charges,” he told me, of being a member of the Oromo secessionist movement.

Zalambessa, Ethiopia, a city on the border with Eritrea, was almost entirely destroyed during the two countries’ war, sparked by a dispute over a tiny parcel of rocky land. Credit Boris Heger/Associated Press

“We have many like him,” Nega said.

Nega put on his jacket to head off in search of diesel fuel for the morning journey to the border. With another rebel comrade from Virginia, we drove down the deserted, lightless streets of Asmara, searching for an open filling station, but the one we found had run out of diesel; Nega would have to return the next morning, delaying his departure for the front lines. When we returned to his home, Nega pointed to a pile of medical supplies in the hallway — bandages, splints, antibiotics, antimalarials — that he was planning to ferry to his fighters, and three cardboard boxes packed with solar cells that would provide some rudimentary electricity in the bush. While in the camps, Nega was dependent on his mobile phone for contact with the outside world, but even that was not guaranteed. “They have shut off phone coverage since the incursion” by the Ethiopians at Tsorona, he told me. “I’ll be out of touch for days.”

When I first met Nega, in late May 2016, the conditions were decidedly more comfortable. After 10 months in Asmara, Nega had flown back to the United States to attend meetings and the graduation of his younger son, Iyassu, from the University of Pennsylvania. Given his deepening involvement in a rebellion against an American ally, it was possible that this would be the last time he could visit the United States. Indeed, Nega, who is not an American citizen, had his State Department-issued “travel document” suspended three years ago, and his application for United States citizenship has been put on indefinite hold. He now travels on an Eritrean passport; together with his green card, it gained him entry into the country — this time. The State Department would not comment on Nega or Ginbot 7, but Nega surmises that the Obama administration does not look favorably on his activities. Still, he insists, “nobody is saying, ‘Back off.’ I think they know that this is not about being against the U.S. We are upholding the basic principles under which the U.S. was established.”

We met over Memorial Day weekend on the terrace of the upscale Café Dupont on Dupont Circle in Washington, joined by his sister Hiwot, who runs a technology start-up in New York, and Iyassu, a 21-year-old former high-school track star who was starting work at a New York investment bank in the fall. Over white wine and chicken salad, the conversation touched on Lin-Manuel Miranda’s commencement address and Nega’s excitement over crossing paths, after the ceremony, with Donald Trump and Vice President Joe Biden. (Trump’s daughter and Biden’s granddaughter were members of Iyassu’s graduating class.) I asked Iyassu if he had reconciled himself to the idea of his father’s new life on the front lines, and he said that he had. “Ultimately he should continue to pursue what he believes in,” he told me. He expressed little interest, though, in visiting his father at his Eritrean rebel camp or delving deeper into the raison d’être of the Ginbot 7 movement. “I just got out of college — my life has its own direction,” he said. “I can’t take time off. … I’m a little bit removed generationally as well.”

The elder Nega is part of a generation of Ethiopians who grew up amid violence and tumult. Over lunch, he recalled what it was like to be a high-school student when a Marxist junta, the Soviet-backed Derg, overthrew Emperor Haile Selassie and ushered in a brutal dictatorship. Nega had grown up privileged, the son of a wealthy entrepreneur, and he watched as his father’s vast commercial corn and soybean farms were seized and security forces began arresting, imprisoning and executing thousands of dissidents, including many students. He and his two older sisters joined a resistance movement called the Ethiopian People’s Revolutionary Party (E.P.R.P.). They went underground, living in safe houses, eluding the police. His eldest sister was later captured and disappeared in the Derg’s prisons. His family searched for her everywhere.

“We had people coming to our house and telling my parents, ‘I saw her at this place.’ My mother used to go out all over looking for her,” Nega recalls. Her former cellmates later told him that she had died in prison, probably by committing suicide with a cyanide capsule that she wore around her neck. “It was common to have cyanide with you because if you were caught, you would be tortured and executed, and through torture you might be forced to betray people,” Nega said. As the crackdown in Addis intensified, the E.P.R.P. sent Nega north to Tigray province, the center of a growing guerrilla war against the Derg; there, he carried out attacks on government forces. In 1978 a power struggle erupted within the E.P.R.P. leadership, and Nega was thrown into prison. He was released one day before guards turned their guns on the remaining prisoners, killing 15. Nega escaped to Sudan, living as a refugee in Khartoum for nearly two years, then obtained political asylum in the United States in 1980.

He earned his bachelor’s degree from the State University of New York at New Paltz, where he also played on the soccer team. While studying for his doctorate at the New School for Social Research, he lived in Brooklyn and wrote his dissertation on the failures of Ethiopian agriculture under the Communist regime. Meanwhile, Ethiopia was sliding deeper into calamity. When the guerrilla movements increased their attacks in Tigray in the mid-1980s, the Derg dictator, Mengistu Haile Mariam, blocked food supplies to the region, creating a devastating famine in which one million people died. Photographs of starving children, disseminated by the news media, catalyzed an international relief effort, Live Aid, and inspired the pop hit “We Are the World,” making Ethiopia a worldwide synonym for hunger. The famine had wound down, and the rebel war was escalating, when Bucknell hired Nega as an assistant professor in 1990. “He never trumpeted his background, the fact that he had been a guerrilla fighter,” says Dean Baker, a former Bucknell colleague who now heads the Center for Economic and Policy Research in Washington.

In 1991, after a decade’s struggle, three rebel groups — the Tigrayan People’s Liberation Front, the Oromo Liberation Front and the Eritrean People’s Liberation Front — defeated the Derg and marched into Addis Ababa. The new government, led by the Tigrayan rebel leader Meles Zenawi, set about rebuilding the war-shattered nation. Nega finally had reason for optimism. He knew Meles well — the prime minister had been in the same university class as his dead sister — and after the Tigrayans consolidated power, Nega obtained a leave of absence from Bucknell and flew with his wife and two sons, both toddlers, back to Addis, determined to help rebuild the country. Nega believed that Meles “had good intentions,” he told me.

But Nega’s enthusiasm for the new government wore off quickly. At Addis Ababa University, where he taught part-time (he had also taken over several of his father’s businesses), administrators cracked down on dissent, banning the student government and the school newspaper. When Nega encouraged his students to press for academic freedoms, police assaulted them and other demonstrators; later, as unrest spread through the city, they shot 41 people dead. Nega spent a month in jail for abetting the protests. “At night I was hearing prisoners being tortured, beaten,” he says.