Ethiopia's military announced last week it had "entered into a war" with the northern region of Tigray, leading to fears of a protracted conflict in Africa's second most populous nation.
Here is what you need to know about the unfolding situation:
Why does Tigray matter?
Tigray is the northernmost region of Ethiopia, bordered to the west by Sudan and Eritrea to the north, and home mostly to the Tigrayan people, who make up six percent of the national population of over 110 million people.
It is one of 10 semi-autonomous federal states organised along ethnic lines.
However, it has long been a centre of power and influence, controlling government for three decades.
In 1975 the Tigray People's Liberation Front (TPLF) launched a prolonged war against the Derg military government in Addis Ababa, which they eventually toppled in 1991.
The TPLF then dominated the alliance that ruled Ethiopia unchallenged before anti-government protests swept Prime Minister Abiy Ahmed to power in 2018 and forced his predecessor Hailemariam Desalegn to step down.
Tigray's battle-hardened and powerful military also took the lead in Ethiopia's war against neighbouring Eritrea over disputed border territory which raged from 1998 to 2000.
This war was only declared officially over in 2018 in a peace-making effort by Abiy that won him the Nobel Peace Prize the following year.
What went wrong?
Under Abiy, who is from the country's largest ethnic group the Oromo, Tigray's leaders have complained of being unfairly targeted in corruption prosecutions, removed from top positions and broadly scapegoated for the country's woes.
The TPLF formally became an opposition party last year when it refused to go along with Abiy's merger of the ruling coalition into a single party, the Prosperity Party.
The feud became more intense after Tigray held its own elections in September, defying Abiy's government which postponed national polls due to the coronavirus pandemic.
Addis Ababa ruled the Tigray government was unlawful and in return Tigray said it no longer recognised Abiy's administration.
The federal government then slashed funding to the region, which the TPLF said was "tantamount to an act of war".
In what the International Crisis Group termed a "sudden and predictable" descent into conflict, Abiy on November 4 said that the TPLF had crossed a "red line" and attacked two federal military bases in Tigray, forcing a "military confrontation".
The TPLF accuses Abiy of concocting the story to justify deploying the military against it.
What is happening now?
A blackout on communications in Tigray has made it hard to verify the situation on the ground.
However, things appeared to have moved fast. Clashes have erupted between Ethiopian and Tigrayan forces, and Abiy claims the airforce has bombed military assets in the region and has promised more airstrikes to come.
Most of the fighting appears concentrated in Tigray's west, including along the border with the Amhara region to the south.
A humanitarian source said that more than 100 fighters had been treated at hospitals in Amhara.
Troops are being moved from other parts of the country towards Tigray, in an apparent attempt to surround Tigrayan forces.
A key question is who is in control of the crucial Northern Command army unit based in Tigray, one of the best equipped in the country.
The TPLF claims the unit has joined the Tigrayan cause, which Addis Ababa denies.
What will happen next?
Alarm is mounting over the prospect of civil war that would pit two powerful armies against each other.
The International Crisis Group warned that unless the fighting was urgently halted, the conflict "will be devastating not just for the country but for the entire Horn of Africa".
Given Tigray has powerful military forces, with an estimated 250,000 troops, a war could be "lengthy and bloody".
The conflict could also further destabilise the diverse nation which has seen multiple outbreaks of ethnic violence in recent years.
"We're working to ensure the war won't come to the centre of the country," said deputy army chief Berhanu Jula. It will end in Tigray, he added.
What wider impact could this have?
There are concerns a conflict in Ethiopia could reverberate across the already fragile Horn of Africa, with an impact in neighbours Somalia, Eritrea, Djibouti and Sudan.
"Immediate, concerted mediation -– local, regional and international -– is needed to avert a descent into a broader crisis," the ICG said.
UN chief Antonio Guterres warned that "the stability of Ethiopia is important for the entire Horn of Africa region."
It is also unclear what this means for Abiy's democratic reforms and his wider peace-making efforts in the region, with elections due next year.
For now, he shows no sign of backing down and has defended his actions, saying the military operation has "clear, limited and achievable objectives -- to restore the rule of law and the constitutional order".