Complex sentence: Behind the side armor was a face-hardened inboard longitudinal splinter bulkhead that was 75 mm thick between the middle and lower decks, but decreased to 25 mm between the middle and upper decks.
Split into simpler sentences: Behind the side armor was a face-hardened inboard longitudinal splinter bulkhead that was 75 mm thick between the middle and lower decks. This decreased to 25 mm between the middle and upper decks.