Complex sentence: Assigned to escort work, she accompanied merchantmen as they plied the Gulf and West Indian shipping lanes, which during the preceding months had gained the dubious distinction of suffering the heaviest losses to U-boat activity in the eastern Atlantic.
Split into simpler sentences: Assigned to escort work, she accompanied merchantmen as they plied the Gulf and West Indian shipping lanes. Lanes which during the preceding months had gained the dubious distinction of suffering the heaviest losses to U-boat activity in the eastern Atlantic.