The Ethiopian Empire, also known as Abyssinia, was in what is now Ethiopia and Eritrea. At its height the empire also included Northern Somalia, Djibouti, Southern Egypt, Eastern Sudan, Yemen and Western Saudi Arabia and existed from approximately 400 BC(beginning of Kingdom of Aksum) until 1974 when the monarchy was overthrown in a coup d'etat. It was in its time the oldest state in the world, and the only native African nation to successfully resist the Scramble for Africa by the colonial powers during the 19th century.
By the 400s BC, the Kingdom of Axum was established on the coast and made itself known as a seafaring people active in the spice trade to India. They became known to the Romans no later than the 30s BC when Augustus conquered Egypt, and it is believed by then the square-rigged Axumite galleys were disdaining the long slow coastal trade route and riding the Monsoon winds to and from India, moreover, having established trading with Rome for goods from inland Africa, the Ethiopians passed the trick on to Roman traders, and probably carried some of their cargos for hire. The sea route also connected with the Silk Road through what is now Pakistan, so the Axumites also aided Rome in obtaining Chinese silk, and by the third century Rome had established trade entrepots in India and the sea route carried virtually all the eastern trade to the consternation of Roman statesmen who decried the flow of bullion out of Rome. Around 300 CE Axum both became Christian, and conquered the neighboring ancient kingdom of Kush. References to that time thereafter began referring to them as an Empire, and they themselves were by then using "Ethiopia" in correspondence. The kingdom spread south and westwards and into the Arabian peninsula over the next few centuries, and generally flourished trading with both the Western Roman Empire or the barbarians who supplanted it and the Byzantine Empire until the Islamic conquest of Egypt ca 640 CE cut the Empire off from European markets. Indications are the Empire turned inland, locating its capital for example further west and expanding its territory in the uplands both to the south and west. References to "Ethiopia" and "Ethopian Christians" are sprinkled through European and Byzantine documents throughout the Early and High Middle Ages, but gradually dwindle, indicating there was some contact over the ensuing centuries after the Muslim conquest, but in general, the Empire went into a slow declining spiral but endured until the last Axumite king failed to produce a male heir, and was overthrown by his son-in-law.
The Habesha reigned with only a few interruptions from 1270 until the late 20th century. It is under this dynasty that most of Ethiopia's modern history is formed. During this time, the empire conquered and incorporated virtually all the peoples within modern Ethiopia and Eritrea. They successfully fought off Arab and Turkish armies and made fruitful contacts with some European powers.
Ethiopia received Eritrea after World War II, which remained within it after the dissolution of the monarchy until Eritrea's separation in 1993.