Complex sentence: He then studied at Columbia for two years where he was a member of the Philolexian Society debating club and then, against his parents' wishes, transferred to the Naval Academy, where he graduated second in his class in 1859.
Split into simpler sentences: He then studied at Columbia for two years, where he was a member of the Philolexian Society debating club. Against the better judgment of his father, Mahan then entered the Naval Academy, where he graduated second in his class in 1859.