Various Semitic-speaking groups from Southwest Arabia began to cross the Red Sea and settle along the coast and in the nearby highlands of Ethiopia during the first millennium BCE and possibly even earlier.

These migrants bring with them their Semitic speech (Sabaean and perhaps others) and script (Old Epigraphic South Arabic) and monumental stone architecture.

A fusion of the newcomers with the indigenous inhabitants produces a culture known as pre-Axumite.

The factors that motivate this settlement in the area are not known, but to judge from subsequent history, commercial activity must have figured strongly.

The port city of Adulis, near modern-day Massawa, is a major regional entrepôt and probably the main gateway to the interior for new arrivals from Southwest Arabia.

The Axumite Red Sea port of Adulis had been described in the anonymous Greek travel book Periplus Maris Erythraei, written in the first century CE, as an “open harbor” containing a settlement of Greco-Roman merchants.

It is through such communities, established for the purposes of trade, that the Monophysite Christianity of the eastern Mediterranean reaches Ethiopia during the reign of Emperor Ezanas.

According to tradition, Frumentius, a Christian youth of Tyre, is attacked and shipwrecked with a companion, Aedesius, on the Ethiopian coast.

The two are taken as captives to …

Alexandrian merchant Cosmas Indicopleustes travels up the Nile.

He happens to be in Adulis on the Red Sea Coast of modern Eritrea at the time (about 520 or 525) when the King of Axum is preparing a military expedition to attack the Jewish Himyarite king Yusuf Dhu Nuwas in Yemen, who had recently been persecuting Christians.

On request of the Axumite king and in preparation for this campaign, he records now-vanished inscriptions such as the Monumentum Adulitanum (which he mistakenly attributes to Ptolemy III Euergetes).

He will ultimately venture as far to the east as Ceylon, become a monk, and, in 550, will write Topographia Christiana to vindicate the biblical account of the world.

Muslims from the Arabian Peninsula destroy Adulis, the port of the Kingdom of Aksum, thus causing the decline of Ethiopian Christianity on the African Red Sea coast.

Adulis Bay is named after the port.

It is thought that the modern town of Zula may be the Adulis of Aksumite times, as Zula may reflect the native name for the Greek "Adulis."

While the scholar Yuri Kobishchanov detailed a number of raids Aksumites made on the Arabian coast (the latest being in 702, when the port of Jeddah was occupied), and argued that Adulis was later captured by the Muslims, which brought to an end Axum's naval ability and contributed to the Aksumite Kingdom's isolation from Cpnstantinople and other traditional allies, the last years of Adulis are a mystery.

Muslim writers occasionally mention both Adulis and the nearby Dahlak Archipelago as places of exile, the evidence suggests that Axum maintained its access to the Red Sea, yet from the seventh century onward suffered a clear decline in fortunes.

In any case, the sea power of Axum wanes and security for the Red Sea falls on other shoulders.