Ethiopia's military is fighting battle-hardened troops in the northern Tigray region, threatening stability around the Horn of Africa.
Here are some facts on the forces:
THE NATIONAL MILITARY: THE ENDF
The Ethiopian National Defence Force (ENDF) has around 140,000 active personnel, the vast majority of them in the army, according to the Janes security data group.
Its troops have been tested by Islamist militants in Somalia and rebel groups in Ethiopia's border regions, as well as the two-decade border war then standoff with Eritrea that only ended in 2018.
Its air force gives it dominance in the skies over Tigray. According to Janes, it has 15 Sukhoi Su-27SK and eight MiG-23ML fighter jets, around 20 Mi-24 and Mi-35 helicopter gunships and a range of air defence and missile systems, as well as scores of Russian T-55 and T-72 tanks.
A senior diplomat working on the Ethiopia crisis said Prime Minister Abiy Ahmed had increasingly fallen back on support from forces from Tigray's southern neighbour Amhara in ground fighting - raising the risk of ethnic violence.
IN THE BALANCE: THE NORTHERN COMMAND
One big question mark lies over the fate of the firepower and personnel of the federal military's powerful Northern Command, headquartered in Tigray's capital Mekelle.
Regional fighters control the headquarters building and have seized heavy weapons, according to a United Nations report seen by Reuters. It is unclear how much of the federal military's hardware was in Tigray when fighting broke out.
Abiy's government says it has carried out air strikes to destroy equipment in the hands of the Tigrayans. "The importance of the armour in Tigray cannot be overstated," a military source in the Horn of Africa told Reuters.
The northern region's ruling Tigray People's Liberation Front (TPLF) says it has taken over the Northern Command's assets. The federal military has acknowledged the loss of a compound, but said its troops remain loyal and have been fighting back.
TIGRAY'S FORCES: THE TPLF
As many as 250,000 soldiers and militia serve under regional commanders in Tigray, according to the International Crisis Group.
It is unclear whether those all count separately from the soldiers under the ENDF's Northern Command and how many are active, but that still amounts to a significant force.
The TPLF says it has air defence systems that have shot down a federal army jet, an assertion dismissed by the military.
The regional force has a formidable history. Tigrayan fighters led the rebel march to drive out the Marxist Derg regime in 1991 and bore the brunt of the Eritrean war.
THE WILD CARD: ERITREA
Over Tigray's northern border, Eritrea's President Isaias Afwerki – a long-time foe of the TPLF - controls a vast standing army which the United States' CIA puts at 200,000 personnel.
His government has dismissed TPLF reports that Eritrean troops have already crossed the border. Any such intervention could tip the Tigray fighting into a regional war.
Eritrea has a system of mandatory military service for all adults which rights groups say amounts to indefinite conscription for many and forces thousands to flee the country.