Complex sentence: Thereafter the conflict consisted essentially of a war between Castile and Portugal, as the naval war in the Atlantic Ocean grew in importance and where the Portuguese fleets overpowered the Castilians in the struggle for the maritime access to the wealth of Guinea.
Split into simpler sentences: Thereafter the conflict consisted essentially of a war between Castile and Portugal. The naval war in the Atlantic grew in importance, where the Portuguese fleets overpowered the Castilians in the struggle for the maritime access to the wealth of Guinea.