On 4 November, Ethiopian Prime Minister Abiy Ahmed ordered a military response to a deadly, "traitorous" attack on federal army camps in Tigray.
Top leaders of the Tigray People's Liberation Front (TPLF) deny the attack occurred and say Mr Abiy used the story as a pretext for an "invasion".
A blackout on communications in Tigray has made it hard to verify the situation on the ground.
What has happened so far?
Things have moved fast, with the army launching air strikes on Tigray arms and fuel depots as heavy fighting played out in the west, where Mr Abiy claims federal forces are now in control.
On Sunday, state media reported they had also seized Alamata, a town in southern Tigray.
Officials report hundreds have been killed, while Amnesty International documented a massacre of "scores, and likely hundreds" of civilians in the town of Mai-Kadra which the United Nations warned could amount to war crimes.
Witnesses told Amnesty that forces backing the TPLF had carried out the killings and that some of the victims were ethnic Amhara, from a neighbouring region which has long had tensions with Tigray.
Ethiopian refugees gather in Qadarif region, eastern Sudan. Source: AP
Tigrayan leader Debretsion Gebremichael told AFP the accusations were "baseless".
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.
On Tuesday, Mr Abiy said military operations would soon enter a "final" phase.
"The three-day deadline for the Tigray regional special forces and militia to hand themselves over to national defence forces instead of being a tool for the greedy junta has expired. Those Tigray special forces and militia who used the three-day deadline are appreciated," he said.
"Since the deadline has been completed, in the coming days the final law enforcement activities will be done."
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 that make up the country.
But 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 UN says Ethiopia's growing conflict has resulted in thousands fleeing from the Tigray region into Sudan. Source: AP
The TPLF then dominated the iron-fisted alliance that ruled Ethiopia unchallenged before anti-government protests swept Mr Abiy 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 peacemaking effort by Mr Abiy that won him the Nobel Peace Prize the following year.
What went wrong?
Under Mr 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 Mr 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 Mr 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".
Mr Abiy, who has continually defended his military operation as crucial to bring the rebellious region to heel, accuses the TPLF of trying to destabilise the country and derail his efforts to transform the country into a true democracy.
He has promised the military operation will be over soon.
How has the conflict spread?
Army chief Berhanu Jula vowed that the war would not spread outside of Tigray, however this is already happening.
On Friday night the TPLF fired rockets at two airports in Ethiopia's Amhara region, south of Tigray.
Thousands of militiamen from Amhara have already joined forces with federal troops fighting in Tigray, and the neighbouring regions have a history of tensions over land that analysts fear could fuel further conflict.
Meanwhile neighbouring Eritrea has also, as feared, been drawn into the conflict.
The TPLF on Sunday admitted firing rockets targeting the airport in the Eritrean capital Asmara, claiming Ethiopian forces are using it to target them, and that Eritrean forces are assisting Mr Abiy's government, which Ethiopia denies.
The International Crisis Group has warned that unless the fighting is urgently halted, the conflict "will be devastating not just for the country but for the entire Horn of Africa".
Ethiopian Orthodox Christians light candles and pray for peace during a church service in the Ethiopian capital Addis Ababa. Source: AP
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.
The fighting has led to a humanitarian crisis, with more than 25,000 fleeing into Sudan, while many more vulnerable people in Tigray have limited access to aid.
