Complex sentence: The Nguyễn defended these lines against numerous Trịnh offensives which lasted (off and on) from 1631 till 1673, when Trịnh Tạc concluded a peace treaty with the Nguyễn Lord, Nguyễn Phúc Tần, and so Vietnam was divided between the two ruling families.
Split into simpler sentences: The Nguyễn defended these lines against numerous Trịnh offensives which lasted (off and on) from 1631 till 1673. In 1673, the Trịnh Lord, Trịnh Tạc concluded a peace treaty with the Nguyễn Lord, Nguyễn Phúc Tần, and so Vietnam was divided between the two ruling families.