Complex sentence: Keith was present later that year at the Battle of Sheriffmuir on 9 November, and subsequently, when the Earl of Mar failed to join up with the English Jacobites and the Catholics in the south, Keith realized that the end of this effort was near.
Split into simpler sentences: Keith was present later that year at the Battle of Sheriffmuir on 9 November. Subsequently, when the Earl of Mar failed to join up with the English Jacobites and the Catholics in the south, Keith realized that the end of this effort was near.