Complex sentence: By 1899 he was a captain in the "Regia Marina" and a close associate of Prince Luigi Amedeo, an Italian duke, nephew of the king of Italy, and experienced mountaineer.
Split into simpler sentences: By 1899 he was a captain in the "Regia Marina" and a close associate of Prince Luigi Amedeo. The prince was an Italian duke, nephew of the king of Italy, and experienced mountaineer.