Complex sentence: The key area of the Arauco War were the valleys around Cordillera de Nahuelbuta where the Spanish designs for this region was to exploit the placer deposits of gold using unfree Mapuche labour from the nearby and densely populated valleys.
Split into simpler sentences: The key area of the Arauco War were the valleys around Cordillera de Nahuelbuta. The Spanish designs for this region was to exploit the placer deposits of gold using unfree Mapuche labour from the densely populated valleys.