The stamp history of Eritrea overlaps to some degree with the postal histories of Italy, which colonized it, and Ethiopia, which annexed it. The Kingdom of Italy first colonized Eritrea in 1890, issuing stamps (Italian issues overprinted) beginning in 1893. The Eritrea of this era had the same name as its modern counterpart but was designated a colony by Italy; when stamps were inscribed with the name of the colony, the phrase used was Colonie Italiane Eritrea, or Eritrea Poste Italiane.
Italian East Africa
In the years leading up to World War 2, Italy collected three different areas — Eritrea, Ethiopia, and Italian Somaliland — into a new colony, Italian East Africa, which began in 1936. (A couple years later, British Somaliland was briefly added; today both Somalilands together comprise Somalia.) Stamps were issued especially for this big new colony, and overprinted Italian stamps were also briefly used.
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British occupation
In 1941, Britain took control of Eritrea from the Italians. They began using British stamps overprinted MEF. Later, from 1948 — 1952, the overprint changed to BMA ERITREA and then BA ERITREA. Britain ceded control of Eritrea when Ethiopia and Eritrea became federated under a UN mandate.
Part of Ethiopia
The Ethiopia-Eritrea federation lasted roughly ten years, until 1961 when Ethiopia declared Eritrea a mere Ethiopian province, annexing the Eritreans and ending their sovereignty. Eritrea bristled at this, and a 30-year struggle for independence commenced. In 1978, an Eritrea liberation group had defiantly Eritrean stamps printed, pushing boldly for independence; these were never legal and were more symbolic than postal, though they are unique items for stamp collectors today who come across them.
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Independent Eritrea stamps
Similar independence-minded stamps were issued in the very early 1990s, and in fact these directly preceded actual Eritrean independence, which was achieved de facto in 1991 and officially in 1993. Since their 1993 independence, Eritrea has continued to issue its own stamps.