"Europeans went in search of a Christian King whom they called Prestor John not knowing the nature or real location of this Christian emperor. From my research they were in fact searching for a white king, expecting that any king in this region who was a champion of Christianity would be white."
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it seems you are researching writings by uropeans. if you do research written by members of, or historians-of the EOC, then you would be quite aware that Lebna Dengel never even received any military assistance from the portugeese. four hundred troops arrived in ethiopia in 1541-Lebna Dengel passed in 1540. Portugeese troops were received by his widow Sable Wangel.

uropeans were not in some search for Prester John (Emporer Dengel). quite the contrary Lebna Dengel initiated communication w/ Portugal and he sought to secure diplomatic alliances w/ other Christian countries.
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"The King that you mentioned may have indeed been the person that fit the time frame of rule, but certainly they were following a MYTH when they went in search of “Prestor John”."
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i dont know what books you are reading, but again 'they' did not go in search of the Emporer, they responded to communiaction that he sent to the portugeese king manuel and his son king joam. this communication was meant to propose an alliance against the turks. this communication DID NOT and WAS NOT at all concerning "oppressing" indigenous ethiopians.

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“A History of the Early Orders of Knighthood and Chivalry”, published at Limpsfield, Surrey, 1978 states that:

“ "Prestor John" is a name which was devised in Western Europe and was applied over many centuries to rulers of 'the East' at various times, this was taken to mean some mythical Christian king somewhere in India, or an unspecified Eastern khanate. Finally, with the Western discovery of the Ethiopian Christian Kingdom, the name was applied to Ethiopia, where the western appellation remained.”
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again i note you base your allegations on info wriiten by so-called white scholars.

i will respond to your other words later