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 Defense 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 a two-year border war with Eritrea followed by an 18-year standoff that only ended in 2018.
Between the ENDF forces and other fighters loyal to the federal government there are an estimated 40-50,000 fighters in Tigray at the moment, two diplomats following the conflict said.
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 defense 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 neighbor 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.
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.
Dozens of ENDF soldiers in the Northern Command remain in their barracks and are taking no part in fighting, a regional security expert told Reuters.
Abiy's government says it has carried out air strikes to destroy equipment in the hands of the Tigrayans. "The importance of the armor in Tigray cannot be overstated," a military source in the Horn of Africa told Reuters.
A series of missile strikes over the last week on Asmara, the capital of neighboring Eritrea, and two sites in Ethiopia - all claimed by the TPLF - show that regional forces have some long-range weapons.
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.
Of these, there are some 30-60,000 effective fighters, one of the diplomats said.
The TPLF says its air defense systems have shot down a federal army jet, an assertion dismissed by the military.
Analysts say they do not yet know what missiles the TPLF used in the strikes on Asmara and targets within Ethiopia but Janes said that Tigray had several S-75 and S-125 surface-to-air missile systems, primarily used for air defense, before fighting broke out.
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, an assertion two diplomats said was highly probable. Any such intervention could tip the Tigray fighting into a regional war.
The TPLF has also accused the federal government of being supported by "non-African actors."
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.