Eritrea, one of the world's most repressive and secretive states, has played a major role in the conflict unleashed last year when Ethiopia launched an offensive against Tigray's dissident leaders.
Soldiers from Eritrea, which borders the northern Ethiopian region, have been accused by residents and rights groups of massacres in several locations that figure among the worst atrocities recorded in the conflict.
Eritrea is a bitter enemy of the Tigray People's Liberation Front (TPLF) -- the party which dominated Ethiopian politics for nearly three decades before falling by the wayside with the appointment of Abiy Ahmed as prime minister in 2018.
- Animosity -
However the current leaders of Eritrea and the TPLF were not always foes.
In 1991 they were allies when a coalition of Ethiopian fighters led by the TPLF ousted dictator Mengistu Hailemariam with the key support of separatist rebels from Eritrea -- then still a part of Ethiopia.
Eritrea gained its independence in 1993, rendering Ethiopia landlocked as it lost access to its crucial Red Sea ports.
Relations between the two rapidly deteriorated over territorial and economic disputes.
In May 1998, Asmara and Addis Ababa went to war over the town of Badme, a conflict marked by trench warfare and large-scale pitched battles.
The war left 80,000 dead and resulted in a nearly two-decade stalemate, fomenting deep distrust and enmity between the leaders of the two countries as the issue of Badme remained unresolved.
- Peace -
Abiy's appointment in 2018 led to a spectacular and unexpected about-turn in relations between Addis Ababa and Asmara.
He had risen through the ranks of the EPRDF governing coalition, in place since 1991, to become the first premier from the country's largest ethnic group, the Oromos.
His appointment came after Oromos and Amharas, the second largest ethnic group, led several years of anti-government protests over their perceived marginalisation, which pushed former premier Hailemariam Desalegn to resign.
Abiy, who embarked on a series of democratic and economic reforms, announced in June 2018 that he wanted to end the border standoff with Eritrea.
Within weeks he and Eritrean President Isaias Afwerki signed a declaration putting an end to the war.
The rapprochement, which won Abiy the Nobel Peace Prize in 2019, placed the powerful TPLF in a difficult position with their enemy to the north now allied with Addis Ababa, with whom tensions had been brewing.
Abiy had begun to sideline the Tigrayan elites whom he saw as a main obstacle to his reforms, and they retreated to their stronghold in Tigray.
The TPLF refused to join Abiy's new ruling Prosperity Party after he dissolved the EPRDF coalition, and held its own elections in defiance of a national postponement due to the coronavirus pandemic.
- Eritrea steps into Tigray -
Soon after Abiy launched his military operation to oust the TPLF in November, widespread reports emerged that Eritrean troops were in the region.
Even the new local authorities appointed by Abiy demanded the Eritreans leave the country.
For months, though, both Addis Ababa and Asmara denied Eritrea's role.
Only this week did Abiy finally tell lawmakers the Eritreans had taken control of "border areas".
Abiy then flew to Asmara to meet with Isaias.
On Friday he announced Eritrea had "agreed to withdraw its forces out of the Ethiopian border." Eritrea has not confirmed this.
Rights groups and Tigrayan residents have described a deep and damaging Eritrean presence.
Amnesty International and Human Rights Watch have accused Eritrean troops of killing hundreds of Tigrayans in a November massacre in the town of Axum.
AFP has separately documented a massacre allegedly carried out by Eritrean troops in the town of Dengolat, also in November.
Roland Marchal, an expert from the Centre for International Research in Paris, said Eritreans were taking advantage by "occupying territory they see as theirs and by forcefully repatriating Eritrean refugees who they have always seen as a potential threat."
Before the conflict, Tigray was home to almost 100,000 Eritrean refugees who had fled the authoritarian country and its system of forced military service.
The Hitsats and Shimelba camps have been reported by the UN and other sources to have been destroyed in the fighting.
Marchal said Eritrea was not just settling scores.
"When you look at what they are doing in Eritrea there is a sense of collective punishment," he said.
"They are busy settling a series of what they see as historic defiances by massacring the civilian population."