http://www.lonelyplanet.com/ethiopia/history

Oromo Migrations & The Jesuits
A new threat to the Ethiopian empire arose in the mid-16th century, filling the power vacuum left behind by the weakened Muslims. The nomadic pastoralists and warrior horsemen of the Oromos (known to the Amharas as Gallas, a pejorative term) began a great migration northwards from what’s now Kenya.

For the next 200 years intermittent armed conflict raged between the empire and the Oromos. For the empire, the Oromo expansion meant loss of territory and vital tax revenue. The Oromos also challenged the old Muslim state; the old city walls seen in Harar today were built in response to Oromo conflicts.

Early in the 17th century the Oromo threat led several Ethiopian emperors to seek an alliance with the Portuguese-backed Jesuits. Two emperors, Za-Dengel and Susenyos, even went as far as conversion to Catholicism. However, imposing Catholicism on their population provoked widespread rebellion. Za-Dengel was overthrown and, in 1629, Susenyos’ draconian measures to convert his people incited civil war.

As many as 32,000 peasants are thought to have lost their lives in the bloodshed that followed, most at the hands of Susenyos’ army. Eventually Susenyos backed down and the Orthodox faith was re-established.

Susenyos’ son and successor, Fasiladas, expelled the meddling Jesuits and forbade all foreigners to set foot in his empire. For nearly 130 years only one European, a French doctor Charles Poncet, was allowed to enter Ethiopia. He famously wrote about Emperor Iyasu’s grandeur in A Voyage to Ethiopia (translation).

Though the Jesuits’ interference had caused great suffering and bloodshed in Ethiopia, they left behind one useful legacy: books. Pero Pais wrote the first serious history of the country. Other writings included detailed accounts of Ethiopia’s cultural, economic and social life.

With the rising Ottoman hold in the east, and the Oromo entrenchment in the south, the political authority of Shoa had become increasingly circumscribed. It was time to relocate the centre of power – again.