NAIROBI (Reuters) - Once again, troops are rushing into a rugged corner of Ethiopia that has been at the heart of momentous events for decades, from war with Eritrea to the toppling of a Marxist dictatorship.This time, it is the federal government sending jets and soldiers against the restive northern Tigray region in an offensive with ethnic undertones that threatens to destabilise both Ethiopia and the wider Horn of Africa. Reuters reporters in the area this week saw trucks full of militia fighters and pickups with machine-guns rushing along windy mountain tracks to the front line in support of federal troops’ offensive against Tigrayan forces. Helicopters flew overhead and some villages were deserted. Conflict and hardship are nothing new to Tigrayans. They were the leading force among rebels who wrested power from the brutal Marxist Derg regime in 1991. They also bore the brunt of a horrific 1998-2000 conflict with Eritrea that saw tens of thousands die in battles over scrubland that drew comparisons with the attrition and trench warfare of World War One. Though accounting for just 5% of the population, Tigrayans dominated politics and the military from 1991 until Prime Minister Abiy Ahmed - an ethnic Oromo - took office in 2018. Their tight grip brought repression of alternative views but also sustained economic growth in a nation that had become a byword for poverty in the West, establishing Ethiopia as a regional powerhouse. Even though Abiy had been a minister and head of a security agency in that government, as premier he condemned past rights abuses and had many leading Tigrayans detained or sidelined. When he merged the ruling coalition into one party last year, Tigray’s political party - the Tigray People’s Liberation Front (TPLF) - refused to join and accused him of undermining the constitution, which enshrines a system of ethnic federalism. In September, Tigray defied Abiy to hold a regional election after the government postponed voting nationwide due to the COVID-19 pandemic. Tigray’s poll was declared illegal and its budget cut. Then last week, Abiy accused the TPLF of attacking a federal military base and launched his offensive.