Even before the war, Mr. Abiy appeared bent on breaking the power of the T.P.L.F., a political group of rebels turned rulers who had dominated Ethiopia for nearly three decades.
A former intelligence officer, Mr. Abiy had once been part of the T.P.L.F.-dominated government. But after he took office in 2018, Mr. Abiy set about draining the T.P.L.F. of its power and influence in Ethiopia, infuriating the leadership.
The T.P.L.F. retreated to its stronghold in Tigray, in the mountainous north of Ethiopia. Tensions grew. In September, the Tigrayans defied Mr. Abiy by going ahead with regional parliamentary elections that had been postponed across Ethiopia because of the coronavirus pandemic. Weeks later, Ethiopian lawmakers cut funding to the region.
On the night of Nov. 3-4, T.P.L.F. forces attacked a federal military base in Tigray and attempted to steal its weapons. The T.P.L.F. has said it had struck preemptively because federal forces were preparing to assault Tigray. Hours later, Mr. Abiy ordered the military offensive into Tigray.
Internet and phone communications were restricted and his cabinet declared a six-month state of emergency in Tigray. But the Ethiopian military, which was dominated by Tigrayan officers, was divided, and fighting erupted between rival military units inside Tigray, according to American officials.
Mr. Abiy bolstered his forces by deploying militia fighters from Amhara, south of Tigray, who swept into western Tigray amid accusations of attacks on civilians. Then troops from Eritrea, Ethiopia’s former enemy, flooded across the border into Tigray from the north to fight alongside Mr. Abiy’s forces.
Federal forces and their allies quickly seized control of Tigray’s capital, Mekelle, and other main towns, but the T.P.L.F. and its armed supporters fled to rural and mountainous areas, where sporadic fighting has continued. Aid workers in Tigray say that heavy shelling and gun battles raged in five different parts of Tigray in early May.