The escalation came after months of feuding between Mr Abiy's government and leaders of Tigray's dominant political party.
For almost three decades, the party was at the centre of power, before it was sidelined by Mr Abiy, who took office in 2018 after anti-government protests.
Mr Abiy pursued reforms, but when Tigray resisted, the political crisis erupted into war.
Remember that powerful party from Tigray? Well, this party - the Tigray People's Liberation Front (TPLF) - was influential in setting up this system.
It was the leader of a four-party coalition that governed Ethiopia from 1991, when a military regime was ousted from power.