Why wasn't Portugal conquered by Spain?

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Why wasn't Portugal conquered by Spain?
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♦Narrated by:
Royal Carter
♦Music Used :
Kevin MacLeod - Drums of the Deep
Kevin MacLeod - BTS Prolog
Nomadic Sunset by Alexander Nakarada @ SerpentSound Studios
Lord, Have Mercie by Karstenholymoly
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♦Sources :
Parker, Geoffrey - The Spanish Armada
Stanley G. Payne - A History of Spain and Portugal
Álvarez Palenzuela, Vicente Ángel. "La guerra civil castellana y el enfrentamiento con Portugal (1475-1479)"
Gerli, E. Michael - Medieval Iberia
#History #Documentary