Sentence:  He had gone into the Japanese <WSD>navy</WSD> , had been trained as an officer , had participated in one or two battles - he never went into detail regarding his military experience - and at the age of twenty-five , quite as a bolt out of the blue , he had walked into the mission as if he belonged here and had become a Christian . 
Meaning of the word marked with <WSD> tags:  an organization of military vessels belonging to a country and available for sea warfare