Complex sentence: England sought to assert more centralized control of the New England colonies in the second half of the 17th century, but the expense of the war with the Dutch ensured only limited English control of the colonies, especially in the Massachusetts Bay Colony.
Split into simpler sentences: England sought to assert more centralized control of the New England colonies in the second half of the 17th century. The expense of the war with the Dutch ensured only limited English control of the colonies, especially in the Massachusetts Bay Colony.